PART III.

RELATIONSHIP OF THE TRILOBITES TO OTHER ARTHROPODA.

It can not be said that the new discoveries of appendagiferous trilobites have added greatly to previous knowledge of the systematic position of the group. Probably none will now deny that trilobites are Crustacea, and more primitive and generalized than any other group in that class. The chief interest at present lies in their relation to the most nearly allied groups, and to the crustacean ancestor.

Trilobites have been most often compared with Branchiopoda, Isopoda, and Merostomata, the present concensus of opinion inclining toward the notostracan branchiopods (Apodidæ in particular) as the most closely allied forms. It seems hardly worth while to burden these pages with a history of opinion on this subject, since it was not until the appendages were fully made out, from 1881 to 1895, that zoologists and palæontologists were in a position to give an intelligent judgment. The present status is due chiefly to Bernard (1894), Beecher (1897, 1900, et seq.), and Walcott (1912, et seq.).

The chief primitive characteristics of trilobites are: direct development from a protaspis common to the subclass; variability in the number of segments, position of the mouth, and type of eyes; and serially similar biramous appendages.

The recent study has modified the last statement slightly, since it appears that in some trilobites there was a modification of the appendages about the mouth, suggesting the initiation of a set of tagmata.

In comparing the trilobites with other Crustacea, the condition of the appendages must be especially borne in mind, for while these organs are those most intimately in contact with the environment, and most subject to modification and change, yet they have proved of greatest service in classification.

Appendages have been found on trilobites from only the Middle Cambrian and Middle and Upper Ordovician, but as the Ordovician was the time of maximum development of the group, it is probable that trilobites of later ages would show degradational rather than progressive changes. All the genera which are known show appendages of the same plan, and although new discoveries will doubtless reveal many modifications of that plan, general inferences may be drawn now with some assurance.

The chief characteristics of the appendages are: first, simple antennules, a primitive feature in all Crustacea, as shown by ontogeny; second, paired biramous appendages, similar to each other all along the body, the youngest and simplest in front of the anal segment, the oldest and most modified on the cephalon. The endobases are retained on all the coxopodites, except possibly, in some species, the anterior ones, and these gnathobases are modified in some genera as mouth-parts, while in others they are similar throughout the series. With these few fundamentals in mind, other Crustacea may be examined for likenesses. The differences are obvious.

Crustacea.

BRANCHIOPODA.

The early idea that the trilobites were closely related to the Branchiopoda was rejuvenated by the work of Bernard on the Apodidæ (1892) and has since received the support of most writers on the subject. Fundamentally, a great deal of the argument seems to be that _Apus_ lies the nearest of any modern representative of the class to the theoretical crustacean ancestor, and as the trilobites are the oldest Crustacea, they must be closely related. Most writers state that the trilobites could not be derived from the Branchiopoda (see, however, Walcott 1912 A), nor the latter from any known trilobite, but both subclasses are believed to be close to the parent stem.

Viewed from the dorsal side, there is very little similarity between any of the branchiopods and the trilobites, and it is only in the Notostraca, with their sessile eyes and depressed form, that any comparison can be made. The chief way in which modern Branchiopoda and Trilobita agree is that both have a variable number of segments in the body, that number becoming very large in _Apus_ on the one hand and _Mesonacis_ and _Pædeumias_ on the other. In neither are the appendages, except those about the mouth, grouped in tagmata. Other likenesses are: the Branchiopoda are the only Crustacea, other than Trilobita, in which gnathobases are found on limbs far removed from the mouth; the trunk limbs are essentially leaf-like in both, though the limb of the branchiopod is not so primitive as that of the trilobite; caudal cerci occur in both groups.

If the appendages be compared in a little more detail, the differences prove more striking than the likenesses.

In the Branchiopoda, the antennules are either not segmented or only obscurely so. In trilobites they are richly segmented.

In Branchiopoda, the antennæ are variable. In the Notostraca they are vestigial, while in the males of the Anostraca they are powerful and often complexly developed claspers. Either condition might develop from the generalized biramous antennas of Trilobita, but the present evidence indicates a tendency toward obsolescence. Claus' observations indicate that the antennæ of the Anostraca are developments of the exopodites, rather than of the endopodites.

The mandibles and maxillæ of the Branchiopoda are greatly reduced, and grouped closely about the mouth. Only the coxopodites of the Trilobita are modified as oral appendages.

The trunk limbs of _Apus_ are supposed to be the most primitive among the Branchiopoda, and comparison will be made with them. Each appendage consists of a flattened axial portion, from the inner margin of which spring six endites, and from the outer, two large flat exites (see fig. 34). This limb is not articulated with the ventral membrane, but attached to it, and, if Lankester's interpretation of the origin of schizopodal limbs be correct, then the limb of _Apus_ bears very little relation to that of the Trilobita. In _Apus_ there is no distinct coxopodite and the endobases which so greatly resemble the similar organs in the Trilobita are not really homologous with them, but are developments of the first endite. Beecher's comparison of the posterior thoracic and pygidial limbs of _Triarthrus_ with those of _Apus_ can not be sustained. Neither _Triarthrus_ nor any other trilobite shows any trace of phyllopodan limbs. Beecher figured (1894 B, pl. 7, figs. 3, 4) a series of endopodites from the pygidium of a young _Triarthrus_ beside a series of limbs from a larval _Apus_. Superficially, they are strikingly alike, but while the endopodites of _Triarthrus_ are segmented, the limbs of _Apus_ are not, and the parts which appear to be similar are really not homologous. The similarity of the thoracic limbs in the two groups is therefore a case of parallelism and does not denote relationship.

Geologically, the Branchiopoda are as old as the Trilobita, and while they did not have the development in the past that the trilobite had, they were apparently differentiated fully as early. Anostraca, Notostraca and Conchostraca, three of the four orders, are represented in the Cambrian by forms which are, except in their appendages, as highly organized as the existing species. Brief notes on the principal Middle Cambrian Branchiopoda follow:

=Burgessia bella= Walcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 177, pl. 27, figs. 1-3; pl. 30, figs. 3, 4.

This is the most strikingly like the modern Branchiopoda of any species described by Walcott from the Middle Cambrian, and invites comparison with _Apus_. The carapace is long, loosely attached to the body, and extends over the greater part of the thorax. The eyes are small, sessile, and close to the anterior margin.

The appendages of the head consist of two pairs of antennæ, and three pairs of slender, jointed legs. Both pairs of antennæ are slender and many-jointed, the antennules somewhat smaller than the antennæ. The exact structure of the limbs about the mouth has not yet been made out, but they are slender, tapering, endopodite-like legs, with at least three or four segments in each, and probably more.

There are eight pairs of thoracic appendages, each limb having the form of the endopodite of a trilobite and consisting of seven segments and a terminal spine. The proximal three segments of each appendage are larger than the outer ones, and have a flattened triangular expansion on the inner side. Walcott also states that "One specimen shows on seven pairs of legs, small, elongate, oval bodies attached near the first joint to the outer side of the leg. These bodies left but slight impression on the rock and are rarely seen. They appear to represent the gills." They are not figured, but taken in connection with the endopodite-like appearance of the segmented limbs, one would expect them to be vestigial exopodites.

A small hypostoma is present on the ventral side, and several of the specimens show wonderfully well the form of the alimentary canal and the hepatic cæca. The main branches of the latter enter the mesenteron just behind the fifth pair of cephalic appendages.

Behind the thorax the abdomen is long, limbless, and tapers to a point. It is said to consist of at least thirty segments.

Compared with _Apus_, _Burgessia_ appears both more primitive and more specialized. The carapace and limbless abdomen are _Apus_-like, but there are very few appendagiferous segments, and the appendages are not at all phyllopodan, but directly comparable with those of trilobites, except, of course, for the uniramous character of the cephalic limbs. A closer comparison may be made with _Marrella_.

=Waptia fieldensis= Walcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 181, pl. 27, figs. 4, 5.

The carapace is short, covering the head and the anterior part of the thorax. The latter consists of eight short segments with appendages, while the six abdominal segments, which are similar to those of the thorax, are without limbs except for the last, which bears a pair of broad swimmerets. The eyes are marginal and pedunculate. The antennules are imperfectly known, but apparently short, while the antennas are long and slender, with relatively few, long, segments. The mandibles appear to be like endopodites of trilobites and show at least six segments. As so often happens in these specimens from British Columbia, the preservation of the other appendages is unsatisfactory. As illustrated (Walcott, 1912 A, pl. 27, fig. 5), both endopodites and exopodites appear to be present, and the shaft of the exopodite seems to be segmented as in _Triarthrus_.

Walcott considers _Waptia_ as a transitional form between the Branchiopoda and the Malacostraca.

=Yohoia tenuis= Walcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 172, pl. 29, figs. 7-13.

This species, though incompletely known, has several interesting characteristics. The head shows, quite plainly in some specimens, the five segments of which it is composed. The eyes are small, situated in a niche between the first and second segments, and are described as being pedunculate. The eight segments of the thorax all show short triangular pleural extensions, somewhat like those of _Remopleurides_ or _Robergia_. The abdomen consists of four cylindrical segments, the last with a pair of expanded caudal rami.

The antennules appear to be short, while the antennas are large, with several segments, ending in three spines, and apparently adapted for serving as claspers in the male. The third, fourth, and fifth pairs of cephalic appendages are short, tapering, endopodite-like legs similar to those of _Burgessia_.

The appendages of the thorax are not well preserved, and there seem to be none on the abdomen.

This species is referred by Walcott to the Anostraca.

=Opabina regalis= Walcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 167, pl. 27, fig. 6; pl. 28, fig. 1.

This most remarkably specialized anostracan is not well enough known to allow comparison to be made with other contemporaneous Crustacea, but it is worthy of mention.

There is no carapace, the eyes are pedunculated, thorax and abdomen are not differentiated, and the telson is a broad, elongate, spatulate plate. There seem to be sexual differences in the form of the anterior cephalic and caudal appendages, but this is not fully established. The most remarkable feature is the long, large, median cephalic appendage which is so suggestive of the proboscis of the recent _Thamnocephalus platyurus_ Packard. The appendages are not well enough preserved to permit a determination as to whether they are schizopodal or phyllopodan.

_Summary._

Walcott referred _Burgessia_ and _Waptia_ to new families under the Notostraca, while _Yohoia_ and _Opabina_ were placed with the Anostraca. Except for the development of the carapace, there is a striking similarity between _Waptia_ and _Yohoia_, serving to connect the two groups.

The Branchiopoda were very highly specialized as early as Middle Cambrian time, the carapace of the Notostraca being fully developed and the abdomen limbless. Some (_Burgessia_) had numerous segments, but most had relatively few. The most striking point about them, however, is that so far as is known none of them had phyllopodan limbs. While the preservation is in most cases unsatisfactory, such limbs as are preserved are trilobite-like, and in the case of _Burgessia_ there can be no possible doubt of the structure. Another interesting feature is the retention by _Yohoia_ of vestiges of pleural lobes. The Middle Cambrian Branchiopoda are more closely allied to the Trilobita than are the modern ones, but still the subclass is not so closely related to that group as has been thought. Modern _Apus_ is certainly much less like a trilobite than has been supposed, and very far from being primitive. The Branchiopoda of the Middle Cambrian could have been derived from the trilobites by the loss of the pleural lobes, the development of the posterior margin of the cephalon to form a carapace, and the loss of the appendages from the abdominal segments. Modern branchiopods can be derived from those of the Middle Cambrian by the modification of the appendages through the reduction of the endopodite and exopodite and the growth of the endites and exites from the proximal segments.

Carpenter (1903, p. 334), from his study of recent crustaceans, has already come to the conclusion that the Branchiopoda are not the most primitive subclass, and this opinion is strengthened by evidence derived from the Trilobita and from the Branchiopoda of the Middle Cambrian.

COPEPODA.

The non-parasitic Eucopepoda are in many ways much nearer to the trilobites than any other Crustacea. These little animals lack the carapace, and the body is short, with typically ten free segments and a telson bearing caudal furcæ. The head is composed of five segments (if the first thoracic segment is really the fused first and second), is often flattened, and lacks compound eyes. Pleural lobes are well developed, but instead of being flattened as in the trilobite, they are turned down at the sides or even incurved. A labrum is present.

The antennules, antennæ, and mandibles are quite like those of trilobites. The antennules are very long and made up of numerous segments. The antennæ are biramous, the junction between the coxopodite and basipodite is well marked, and the endopodite consists of only two segments.

The mandibles are said to "retain more completely than in any other Crustacea the form of biramous swimming limbs which they possess in the nauplius." The coxopodites form jaws, while both the reduced endopodite and exopodite are furnished with long setæ. The maxillulæ are also biramous, but very different in form from those of the trilobite, and the maxillæ are phyllopodan.

The first thoracic limb is uniramous and similar to the maxillæ, but the five following pairs are biramous swimming legs with coxopodite, basipodite, exopodite, and endopodite. Both the exopodite and endopodite are shorter than in the trilobites, but bear setæ and spines.

The last pair of thoracic limbs are usually modified in the male into copulatory organs. In some females they are enlarged to form plates for the protection of the eggs, in others they are unmodified. In still others they are much reduced or disappear. The abdomen is without appendages.

The development in Copepoda is direct, by the addition posteriorly to the larval form (nauplius) of segments, and the appendages remain nearly unmodified in the adult.

Altogether, the primitive Copepoda seem much more closely allied to the Trilobita than any other modern Crustacea, but unfortunately no fossil representative of the subclass has been found. This is not so surprising when one considers the habits and the habitat of most of the existing species. Many are parasitic, many pelagic in both fresh and marine waters, and many of those living on the bottom belong to the deep sea or fresh water. Most free-living forms are minute, and all have thin tests.

The eyes of copepods are of interest, in that they suggest the paired ocelli of the Harpedidæ and Trinucleidæ. In the Copepoda there are, in the simplest and typical form of these organs, three ocelli, each supplied with its own nerve from the brain. Two of these are dorsal and look upward, while the third is ventral. In some forms the dorsal ocelli are doubled, so that five in all are present (cf. some species of Harpes with three ocelli on each mound). In some, the cuticle over the dorsal eyes is thickened so as to form a lens, as appears to be the case in the trilobites. These peculiar eyes may be a direct inheritance from the Hypoparia.

ARCHICOPEPODA.

Professor Schuchert has called my attention to the exceedingly curious little crustacean which Handlirsch (1914) has described from the Triassic of the Vosges. Handlirsch erected a new species, genus, family, and order for this animal, which he considered most closely allied to the copepods, hence the ordinal name. _Euthycarcinus kessleri_, the species in question, was found in a clayey lens in the Voltzia sandstone (Upper Bunter). Associated with the new crustacean were specimens of _Estheria_ only, but in the Voltzia sandstone itself land plants, fresh and brackish water animals, and occasionally, marine animals are found. The clayey lens seems to have been of fresh or brackish water origin.

All of the specimens (three were found) are small, about 35 mm. long without including the caudal rami, crushed flat, and not very well preserved. The head is short, not so wide as the succeeding segments, and apparently has large compound eyes at the posterior lateral angles. The thorax consists of six segments which are broader than the head or abdomen. The abdomen, which is not quite complete in any one specimen, is interpreted by Handlirsch as having four segments in the female and five in the male. Least satisfactory of all are traces of what are interpreted by the describer as a pair of long stiff unsegmented cerci or stylets on the last segment.

The ventral side of one head shield shows faint traces of several appendages which must have presented great difficulty in their interpretation. A pair of antennules appear to spring from near the front of the lower surface, and the remainder of the organs are grouped about the mouth, which is on the median line back of the center. Handlirsch sees in these somewhat obscure appendages four pairs of biramous limbs, antennæ, mandibles, maxillulæ, and maxillæ, both branches of each consisting of short similar segments, endopodites and exopodites being alike pediform.

Each segment of the thorax has a pair of appendages, and those on the first two are clearly biramous. The endopodites are walking legs made up of numerous short segments (twelve or thirteen according to Handlirsch's drawing), while the exopodite is a long breathing and rowing limb, evidently of great flexibility and curiously like the antennules of the same animal. The individual segments are narrow at the proximal end, expand greatly at the sides, and have a concave distal profile. A limb reminds one of a stipe of _Diplograptus_. Both branches are spiniferous.

No appendages are actually present on the abdomen, but each segment has a pair of scars showing the points of attachment. From the small size of these, it is inferred that the limbs were poorly developed.

This species is described in so much detail because, if it is a primitive copepod, it has a very important bearing on the ancestry of that group and is the only related form that has been found fossil.

The non-parasitic copepods have typically ten (eleven) free segments, including the telson, and the four abdominal segments are much more slender than the six in front of them. In this respect the agreement is striking, and the presence of five pairs of appendages in the head and six free segments in the thorax is a more primitive condition than in modern forms where the first two thoracic segments are apparently fused (Calman, 1909, p. 73).

The large compound eyes of this animal are of course not present in the copepods, but as vestiges of eyes have been found in the young of _Calanus_, it is possible that the ancestral forms had eyes.

The greatest difficulty is in finding a satisfactory explanation of the appendages. The general condition is somewhat more primitive than in the copepods, for all the appendages are biramous, while in the modern forms the maxillipeds are uniramous and the sixth pair of thoracic appendages are usually modified in the male as copulatory organs. In the copepods the modification is in the direction of reduction, both endopodites and exopodites usually possessing fewer segments than the corresponding branches in the trilobites. The endopodite of _Euthycarcinus_, on the contrary, possesses, if Handlirsch's interpretation is correct, twice as many segments as the endopodite of a trilobite. If the Copepoda are descended from the trilobites, as everything tends to indicate, then _Euthycarcinus_ is certainly not a connecting link. The only truly copepodan characteristic of this genus is the agreement in number and disposition of free segments. The division into three regions instead of two, the compound eyes, and the structure of the appendages are all foreign to that group.

With the Limulava fresh in mind, one is tempted to compare _Euthycarcinus_ with that ancient type. The short head and large marginal eyes recall _Sidneyia_, and the grouping of the appendages about the mouth also suggests that genus and _Emeraldella_. In the Limulava likewise there is a contraction of the posterior segments, although it is behind the ninth instead of the sixth. There is no likeness in detail between the appendages of the Limulava and those of _Euthycarcinus_, but the composite claws of _Sidneyia_ show that in this group there was a tendency toward the formation of extra segments.

If this fossil had been found in the Cambrian instead of the Triassic, it would probably have been referred to the Limulava, and is not at all impossible that it is a descendant from that group. As a connecting link between the Trilobita and Copepoda it is, however, quite unsatisfactory.

OSTRACODA.

The bivalved shell of the Ostracoda gives to this group of animals an external appearance very different from that of the trilobites, but the few appendages, though highly modified, are directly comparable. The development, although modified by the early appearance of the bivalved shell within which the nauplius lies, is direct. Imperfect compound eyes are present in one family.

The antennules are short and much modified by functioning as swimming, creeping, or digging organs. They consist of eight or less segments. The antennas are also locomotor organs, and in most orders are biramous. The mandibles are biramous and usually with, but sometimes without, a gnathobase. The maxillulæ are likewise biramous but much modified.

The homology of the third post-oral limb is in question, some considering it a maxilla and others a maxilliped. It has various forms in different genera. It is always much modified, but exopodite and endopodite are generally represented at least by rudiments. The fourth post-oral limb is a lobed plate, usually not distinctly segmented, and the fifth a uniramous pediform leg. The sixth, if present at all, is vestigial.

Very little comparison can be made between the Ostracoda and Trilobita, other than in the ground-plan of the limbs, but the presence of biramous antennæ is a primitive characteristic.

CIRRIPEDIA.

Like the ostracod, the adult cirriped bears little external resemblance to the trilobite. The form of the nauplius is somewhat peculiar, but it has the typical three pairs of appendages, to which are added in the later metanauplius stages the maxillæ and six pairs of thoracic appendages. In the adult, the antennules, which serve for attachment of the larva, usually persist in a functionless condition, while the antennas disappear. The mandibles, maxillulæ, and maxillæ are simple and much modified to form mouth parts, and the six pairs of thoracic appendages are developed into long, multisegmented, biramous appendages bearing numerous setæ which serve for catching prey. Paired eyes are present in later metanauplius stages, but lost early in the development. The relationship to the trilobite evidently is not close.

MALACOSTRACA.

_1. Phyllocarida._

The oldest malacostracans whose appendages are known are species of _Hymenocaris_. One, described as long ago as 1866 by Salter, has what seem to be a pair of antennæ and a pair of jaw-like mouth-parts. Another more completely known species has recently been reported by Walcott (1912 A, p. 183, pl. 31, figs. 1-6). This latter form is described as having five pairs of cephalic appendages: a pair of minute antennules beside the small pedunculated eyes, a pair of large uniramous antennæ, slender mandibles and maxillulæ, and large maxillæ composed of short stout segments. There are eight pairs of biramous thoracic limbs, the exopodites setiferous, the endopodites composed of short wide segments and ending in terminal claw-like spines. These appendages are like those of trilobites.

_Hymcnocaris_ belongs to the great group of extinct ceratocarid Crustacea which are admitted to the lowest of the malacostracan orders, Phyllocarida, because of their resemblance to _Nebalia_, _Paranebalia_, _Nebaliopsis_, and _Nebaliella_, the four genera which are at present living. The general form of the recent and fossil representatives of the order is strikingly similar. The chief outward difference is that in many of the fossils the telson is accompanied by two furcal rami, while in the modern genera it is simple. It now becomes possible to make some comparison between the appendages of _Hymcnocaris_ of the Middle Cambrian and the Nebaliidæ of modern seas.

In both there are five pairs of cephalic and eight of thoracic appendages, while those of the abdomen of Hymenocaris are not known.

In both, the antennules are less developed than the antennæ. In the Nebaliidæ the antennules show evidence of having been originally double (they are obviously so in the embryo), while they are single in _Hymcnocaris_. In both, the antennæ are simple. The remaining cephalic organs are too little shown by the specimen from the Middle Cambrian to allow detailed comparison. The mandibles, maxillulæ, and maxillæ of _Nebalia_ are, however, of types which could be derived from the trilobite.

In three of the genera of the Nebaliidæ, the eight pairs of thoracic limbs are all similar to one another, though those of the genera differ. All are biramous. The limbs of _Hymcnocaris_ can apparently be most closely correlated with those of _Nebalia antarctica_, in which the endopodite consists of short flattened segments, and the exopodite is a long setiferous plate. Epipodites are present in both _Nebalia_ and _Hymcnocaris_.

So far as the appendages of _Hymenocaris_ are known, they agree very well with those of the Nebaliidæ, and since they are of the trilobite type, it may safely be stated that the Trilobita and Malacostraca are closely related.

_2. Syncarida._

Walcott (1918, p. 170) has compared the limbs of _Neolenus_ with those of the syncarid genera _Anaspides_ and _Koonunga_. These are primitive Malacostraca without a carapace, but as they have a compressed test and _Anaspides_ has stalked eyes, their gross anatomy does not suggest the trilobite. The thoracic appendages are very trilobite-like, since the endopodite has six segments (in _Anaspides_) and a multisegmented setiferous exopodite. The coxopodites, except of the first thoracic segment, do not, however, show endobases, and those which are present are peculiar articulated ones. The cephalic appendages are specialized, and the antennules double as in most of the Malacostraca. External epipodites are very numerous on the anterior limbs.

This group extends back as far as the Pennsylvanian and had then probably already become adapted to fresh-water life. It may be significant that the Palæozoic syncarids appear to have lacked epipodites. While differing very considerably from the Trilobita, the Syncarida could have been derived from them.

_3. Isopoda._

Since the earliest times there has been a constant temptation to compare the depressed shields of the trilobites with the similar ones of isopods. Indeed, when _Scrolls_ with its Lichadian body was first discovered about a hundred years ago, it was thought that living trilobites had been found at last. The trilobate body, cephalic shield, sessile eyes, abdominal shield, and pleural extensions make a wonderful parallel. This similarity is, however, somewhat superficial. The appendages are very definitely segregated in groups on the various regions of the body, and while the pleopods are biramous, the thoracic legs are without exopodites (except in very early stages of development of one genus). The Isopoda arose just at the time of the disappearance of the Trilobita, and there seems a possibility of a direct derivation of the one group from the other. It should be pointed out that while the differences of Isopoda from Trilobita are important, they are all of a kind which could have been produced by the development from a trilobite-like stock. For example:

Isopoda have a definite number of segments. There is less variation in the number of segments among the later than the earlier trilobites.

Isopoda have no facial suture. In at least three genera of trilobites the cheeks become fused to the cranidium and the sutures obliterated.

Isopoda have one or two segments of the thorax annexed to the head. While this is not known to occur in trilobites, it is possible that it did.

Most Isopoda have a fairly stiff ventral test. The ventral membrane of trilobites would probably have become stiffened by impregnation of lime if the habit of enrollment had been given up.

In Isopoda the antennæ are practically uniramous sensory organs. The second cephalic appendages of trilobites are capable of such development through reduction of the exopodite.

In the Isopoda the coxopodites are usually fused with the body, remaining as free, movably articulated segments only in a part of the thoracic legs of one suborder, the Asellota. Endobases are entirely absent. This is of course entirely unlike the condition in Trilobita, but a probable modification.

In Isopoda there is a distinct grouping of the appendages, with specialization of function. The trilobites show a beginning of tagmata, and such development would be expected if evolution were progressive.

In both groups, development from the embryo is direct. Rudiments of exopodites of thoracic legs have been seen in the young of one genus.

The oldest known isopod is _Oxyuropoda ligioides_ Carpenter and Swain (Proc. Royal Irish Acad., vol. 27, sect. B, 1908, p. 63, fig. 1), found in the Upper Devonian of County Kilkenny, Ireland. The appendages are not known, but the test is in some ways like that of a trilobite. The thorax, abdomen, and pygidium are especially like those of certain trilobites, and there is no greater differentiation between thorax and abdomen than there is between the regions before and behind the fifteenth segment of a _Pædeumias_ or _Mesonacis_. The anal segment is directly comparable to the pygidium of a _Ceraurus_, the stiff unsegmented uropods being like the great lateral spines of that genus.

The interpretation of the head offered by Carpenter and Swain is very difficult to understand, as their description and figure do not seem to agree. What they consider the first thoracic segment (fused with the head) seems to me to be the posterior part of the cephalon and it shows at the back a narrow transverse area which is at least analogous to the nuchal segment of the trilobite. If this interpretation can be sustained, _Oxyuropoda_ would be a very primitive isopod in which the first thoracic segment (second of Carpenter and Swain) is still free. According to the interpretation of the original authors, the species is more specialized than recent Isopoda, as they claim that two thoracic segments are fused in the head. The second interpretation was perhaps made on the basis of the number of segments (nineteen) in a recent isopod.

=Marrella splendens= WALCOTT.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192, pls. 25, 26.

Among the most wonderful of the specimens described by Doctor Walcott is the "lace crab." While the systematic position was not satisfactorily determined by the describer, it has been aptly compared to a trilobite. The great nuchal and genal spines and the large marginal sessile eyes, coupled with the almost total lack of thoracic and abdominal test, give it a bizarre appearance which may obscure its real relationships.

The cephalon appears to bear five pairs of appendages, antennules, and antennæ, both tactile organs with numerous short segments, mandibles, and first and second maxillæ. The last three pairs are elongate, very spinose limbs, of peculiar appearance. They seem to have seven segments, but are not well preserved. These organs are attached near the posterior end of the labrum.

There are twenty-four pairs of biramous thoracic appendages, which lack endobases. The endopodites are long and slender, with numerous spines; the exopodites have narrow, thin shafts, with long, forward pointed setæ. The anal segment consists of a single plate.

Further information about this fossil will be eagerly awaited. None of the illustrations so far published shows biramous appendages on the cephalon. This, coupled with the presence of tactile antennæ, makes its reference to the Trilobita impossible, but the present interpretation indicates that it was closely allied to them.

_Restoration of Marrella._

(Text fig. 32.)

The accompanying restoration of the ventral surface of _Marrella_ is a tentative one, based on Doctor Walcott's description and figures. The outline is taken from his plate 26, figure 1; the appendages of the head from plate 26, figures 1-3, 5, and plate 25, figures 2, 3; the endopodites, shown on the left side only, from figures 3 and 6, plate 25. I have not studied actual specimens, and the original description is very incomplete. The restoration is therefore subject to revision as the species becomes better known.

Arachnida.

No attempt will be made to pass in review all of the subclasses of the arachnids. Some of the Merostomata are so obviously trilobite-like that it would seem that their relationship could easily be proved. The task has not yet been satisfactorily accomplished, however, and new information seems only to add to the difficulties.

So far as I know, the Araneæ have not previously been compared directly with trilobites, although such treatment consists merely in calling attention to their crustacean affinities, as has often been done.

Carpenter's excellent summary (1903, p. 347) of the relationship of the Arachnida to the trilobites may well be quoted at this point:

The discussion in a former section of this essay on the relationship between the various orders of Arachnida led to the conclusion that the primitive arachnids were aquatic animals, breathing by means of appendicular gills. Naturally, therefore, we compare the arachnids with the Crustacea rather than with the Insecta. The immediate progenitors of the Arachnida appear to have possessed a head with four pairs of limbs, a thorax with three segments, and an abdomen with thirteen segments and' a telson, only six of which can be clearly shown by comparative morphology to have carried appendicular gills. But embryological evidence enables us to postulate with confidence still more remote ancestors in which the head carried well developed compound eyes and five pairs of appendages, while it may be supposed that all the abdominal segments, except the anal, bore limbs. In these very ancient arthropods, all the limbs, except the feelers, had ambulatory and branchial branches; and one important feature in the evolution of the Arachnida must have been the division of labour between the anterior and posterior limbs, the former becoming specialized for locomotion, the latter for breathing. Another was the loss of feelers and the degeneration of the compound eyes. Thus we are led to trace the Arachnida (including the Merostomata and Xiphosura) back to ancestors which can not be regarded as arachnids, but which were identical with the primitive trilobites, and near the ancestral stock of the whole crustacean class.

TRILOBITES NOT ARACHNIDA.

While no one having any real knowledge of the Trilobita has adopted Lankester's scheme of the inclusion of the group as the primitive grade in the Arachnida, reference to it may not be amiss. This theory is best set forth in the Encyclopædia Britannica, Eleventh Edition, under the article on Arachnida. It is there pointed out that the primitive arachnid, like the primitive crustacean, should be an animal without a fixed number of somites, and without definitely grouped tagmata. As Lankester words it, they should be anomomeristic and anomotagmatic. The trilobites are such animals, and he considers them Arachnida and not Crustacea for the following reasons:

Firstly and chiefly, because they have only one pair (apart from the eyes) of pre-oral appendages. "This fact renders their association with the Crustacea impossible, if classification is to be the expression of genetic affinity inferred from structural coincidence."

Secondly, the lateral eyes resemble no known eyes so closely as the lateral eyes of _Limulus_.

Thirdly, the trilobation of the head and body, due to the expansion and flattening of the sides or pleura, is like that of _Limulus_, but "no crustacean exhibits this trilobite form."

Fourthly, there is a tendency to form a pygidial or telsonic shield, "a fusion of the posterior somites of the body, which is precisely identical in character with the metasomatic carapace of _Limulus_." No crustacean shows metasomatic fusion of segments.

Fifthly, a large post-anal spine is developed "in some trilobites" (he refers to a figure of _Dalmanites_).

Sixthly, there are frequently lateral spines on the pleura as in _Limulus_. No crustacean has lateral pleural spines.

These points may be taken up in order.

1. If trilobites have one appendage-bearing segment in front of the mouth, they are Arachnida; if two, Crustacea. This is based on the idea that in the course of evolution of the Arthropoda, the mouth has shifted backward from a terminal position, and that as a pair of appendages is passed, they lose their function as mouth-parts and eventually become simple tactile organs. Thus arise the cheliceræ of most arachnids, and the two pairs of tactile antennæ of most Crustacea. This theory is excellent, and the rule holds well for modern forms, but as shown by the varying length of the hypostoma in different trilobites, the position of the mouth had not become fixed in that group. In some trilobites, like _Triarthrus_, the gnathobases of the second pair of appendages still function, but in all, so far as known, the mouth was back of the points of attachment of at least two pairs of appendages, and in some at least, back of the points of attachment of four pairs. As pointed out in the case of _Calymene_ and _Ceraurus_, the trilobites show a tendency toward the degeneration of the first and second pairs of biramous appendages, particularly of the gnathobases. They are in just that stage of the backward movement of the mouth when the function of the antennæ as mandibles has not yet been lost. If the presence of functional gnathobases back of the mouth, rather than the points of attachment in front of the mouth, is to be the guide, then Triarthrus might be classed as an arachnid and _Calymene_ and _Isotelus_ as crustaceans. In other words, the rule breaks down in this primitive group.

2. Superficially, the eyes of some trilobites do look like those of _Limulus_, but how close the similarity really was it is impossible to say. The schizochroal eyes were certainly very different, and Watase and Exner both found the structure of the eye of the trilobite unlike that of _Limulus_.

3. The importance of the trilobate form of the trilobite is very much overestimated. It and the pygidium are due solely to functional requirements. The axial lobe contained practically all the vital organs and the side lobes were mechanical in origin and secondarily protective. That the crustacean is not trilobate is frequently asserted by zoologists, yet every text-book contains a picture of a segment of a lobster with its axial and pleural lobes. It is a fundamental structure among the Crustacea, obscured because most of them are compressed rather than depressed.

4. The pygidium of trilobites is compared with the metasomatic shield of _Limulus_. No homology, if homology is intended, could be more erroneous. The metasomatic shield of _Limulus_ is, as shown by ontogeny and phylogeny, formed by the fusion of segments formerly free, and includes the segments between the cephalic and anal shields, or what would be known as the thorax of a trilobite. No trilobite has a metasomatic shield. The pygidium of a trilobite, as shown by ontogeny, is built up by growth in front of the anal region, and since the segments were never free, it can not strictly be said to be composed of fused segments. Some Crustacea do form a pygidial shield, as in certain orders of the Isopoda.

5. The post-anal spine of Dalmanites and some other trilobites is similar to that of _Limulus_, but this seems a point of no especial significance. That a similar spine has not been developed in the Crustacea is probably due to the fact that they do not have the broad depressed shape which makes it so difficult for a _Limulus_ to right itself when once turned on its back. Relatively few trilobites have it, and it is probably correlated with some special adaptation.

6. There is nothing among the trilobites comparable to the movable lateral spines of the metasoma of _Limulus_.

While, as classifications are made up, the Trilobita must be placed in the Crustacea rather than the Arachnida, there is no reason why both the modern Crustacea and the Arachnida should not be derived from the trilobites.

MEROSTOMATA.

It has been a custom of long standing to compare the trilobite with _Limulus_. Packard (1872) gave great vitality to the theory of the close affinity of the two when he described the so-called trilobite-stage in the development of _Limulus polyphemus_. His influence on Walcott's ideas (1881) is obvious. Lankester has gone still further, and associated the Trilobita with the Merostomata in the Arachnida.

The absence of antennules at any stage in development allies _Limulus_ so closely with the Arachnida and separates it so far from the Trilobita that in recent years there has been a tendency to give up the attempt to prove a relationship between the merostomes and trilobites, especially since Clarke and Ruedemann, in their extensive study of the Eurypterida, found nothing to indicate the crustacean nature of that group. A new point of view is, however, presented by the curious _Sidneyia inexpectans_ and _Emeraldella brocki_ described by Walcott from the Middle Cambrian.

=Sidneyia inexpectans= Walcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 21, pl. 2, fig. 1 (not figs. 2, 3); pls. 3-5; pl. 6, fig. 3; pl. 7, fig. 1.

The body of this animal is elongate, somewhat eurypterid-like, but with a broad telson supplied with lateral swimmerets. The cephalon is short, with lateral compound eyes. The trunk consists of eleven segments, the anterior nine of which are conspicuously wider than the two behind them, and the telson consists of a single elongate plate.

On the ventral side of the head there is a large hypostoma and five, pairs of appendages. The first pair are multisegmented antennules. The second pair have not been adequately described. The third are large, complex claws, and the fourth and fifth suggest broad, stocky endopodites. Broad gnathobases are attached to the coxopodites of the third to fifth pairs of appendages and form very strong jaws.

The first nine segments of the thorax have one pair each of broad filiform branchial appendages, suggestive of the exopodites of trilobites, but no endopodites have been seen. The tenth and eleventh segments seem to lack appendages entirely.

=Emeraldella brocki= Walcott.

Illustrated: _Sidneyia inexpectans_ Walcott _partim_, Smithson. Misc. Coll., vol. 57, 1911, pl. 2, figs. 2, 3 (not fig. 1);--Ibid., 1912, p. 206, text fig. 10.

_Emeraldella brocki_ Walcott, Ibid., 1912, p. 203, pl. 30, fig. 2; text fig. 8;--Ibid., vol. 67, 1918, p. 118 (correction).

_Emeraldella_ has much the same shape as _Sidneyia_ and the same number of segments, but instead of a broad flat telson, it has a long _Limulus_-like spine. The cephalon is about as wide as long, and eyes have not yet been seen. The body consists of eleven segments and a telson (Walcott says twelve and a telson but shows only eleven in the figures). Nine of the segments, as in _Sidneyia_, are broad, the next two narrow.

The ventral side of the cephalon has a long hypostoma, and five pairs of appendages. The first pair are very long multi segmented antennules and the next four pairs seem to be rather slender, spiniferous, jointed endopodites. Whether or not gnathobases were present is not shown by the figures, but owing to the long hypostoma the appendages are grouped about the mouth. All the segments of the body, unless it were the telson, seem to have borne appendages. On the anterior end, they were clearly biramous (1912, p. 206, text fig. 10), and that they were present along the body is shown by figure 2, plate 30, 1912.

The present state of knowledge of both these peculiar animals leaves much to be desired. The indications are that the cephalic appendages are not biramous, and that only one pair of antennæ, the first, are developed as tactile organs. The thoracic appendages of _Emeraldella_ are biramous, and also possibly those of _Sidneyia_. In the latter, the last two abdominal segments seem to have been without appendages, while in _Emeraldella_ at least one branch of each appendage, and possibly both, is retained.

These animals, which may be looked upon as the last survivors of an order of pre-Cambrian arthropods, have the appearance of an eurypterid, but their dominant characteristics are crustacean. The features which suggest the Eurypterida are: elongate, obovate, non-trilobate, tapering body; telson-like posterior segment; marginal, compound, sessile eyes; claw-like third cephalic appendages; and, more particularly, the general resemblance of the test to that of an eurypterid like _Strabops_. In form, _Sidneyia_ agrees with the theoretical prototype of the Eurypterida reconstructed by Clarke and Ruedemann (Mem. 14, N. Y. State Mus., vol. 1, 1912, p. 124) in its short wide head with marginal eyes, and its undifferentiated body. There is, moreover, no differentiation of the postcephalic appendages.

The crustacean characteristics are seen in the presence of five, instead of six, pairs of appendages on the head, the first of which are multisegmented antennules, and in the biramous appendages on the body of _Emeraldella_. It should be noted that these latter are typically trilobitic, each consisting of an endopodite with six segments and a setiferous exopodite.

Clarke and Ruedemann (1912, p. 406) have discussed _Sidneyia_ briefly, and conclude:

It seems to us probable that the Limulava [_Sidneyia_ and _Amiella_] as described are not eurypterids but constitute a primitive order, though exhibiting some remarkable adaptive features. This order possibly belongs to the Merostomata, but is distinctly allied to the crustaceans in such important characters as the structure of the legs and telson, and is therefore much generalized.

The specialization of _Sidneyia_ consists in the remarkable development of a highly complex claw on each of the third cephalic appendages, and in the compound tail-fin, built up of the last segment and one or more pairs of swimmerets. These two characteristics seem to preclude the possibility of deriving the eurypterids from _Sidneyia_ itself, but it seems entirely within reason that they may have been derived from another slightly less specialized member of the same order.

That _Sidneyia_ is descended from any known trilobite seems highly improbable, but that it was descended from the same ancestral stock as the trilobites is, I believe, indicated by the presence of five pairs of appendages on the cephalon and trilobitic legs on the abdomen.

=Molaria= and =Habelia.=

Other so-called Merostomata found by Walcott in the Middle Cambrian are the genera _Molaria_ and _Habelia_, both referred to the Cambrian family Aglaspidæ. These genera seem to conform with _Aglaspis_ of the Upper Cambrian in having a trilobite-like cephalon without facial sutures, a trilobite-like thorax of a small but variable (7-12) number of segments, and a _Limulus_-like telson. Neither of them has yet been fully described or figured, but (Walcott 1912 A, p. 202) _Habelia_ appears to have five pairs of cephalic appendages, the first two pairs of which are multisegmented antennæ. The thoracic appendages are likewise none too well known, but they appear to have been biramous. The endopodites are better preserved than the exopodites, but in at least one specimen of _Molaria_ the exopodites are conspicuous.

If these genera are properly described and figured, their appendages are typically crustacean, and fundamentally in agreement with those of _Marrella_. The relation to the Trilobita is evidently close, the principal differences being the absence of facial sutures and the presence of true antennæ. I am therefore transferring the Aglaspidæ from the Merostomata to a new subclass under the Crustacea.

ARANEÆ.

The spiders have the head and thorax fused, the abdomen unsegmented except in the most primitive suborder, and so appear even less trilobite-like than the insects. The appendages likewise are highly specialized. The cephalothorax bears six pairs of appendages, the first of which are the pre-oral cheliceræ, while behind the mouth are the pedipalpi and four pairs of ambulatory legs. The posterior pairs of walking legs belong to the thorax, but the anterior ones are to be homologized with the maxillæ of Crustacea, so that the spiders are like the trilobites in having functional walking legs on the head.

The chief likenesses are, however, seen in the very young. On the germ band there appear a pair of buds in front of the rudiments of the cheliceræ which later unite to form the rostrum of the adult. At the time these buds appear, the cheliceræ are post-oral, but afterward move forward so that both rostrum and cheliceræ are in front of the mouth. The rostrum is therefore the product of the union of the antennules, and the cheliceræ are to be homologized with the antennæ. There seems to be some doubt about the homology of the pedipalps with the mandibles, as at least one investigator claims to have found rudiments of a segment between the one bearing the cheliceræ and that with the pedipalps.

Jaworowski (Zool. Anzeiger, 1891, p. 173, fig. 4) has figured the pedipalp from the germ band of _Trochosa singoriensis_, and called attention to the fact that it consists of a coxopodite and two segmented branches which may be interpreted as exopodite and endopodite. He designated as exopodite the longer branch which persists in the adult, but since the ambulatory legs of Crustacea are endopodites, that would seem a more likely interpretation. As the figure is drawn, the so-called endopodite would appear to spring from the proximal segment of the "exopodite." If the two terms were interchanged, the homology with the limb of the trilobite or other crustacean would be quite perfect.

In the young, the abdomen is segmented and the anterior segments develop limb-buds, the first pair of which become the lung books and the last two pairs the spinnerets of the adult. There seems to be some question about the number of segments. Montgomery (Jour. Morphology, vol. 20, 1909, p. 337). reviewing the literature, finds that from eight to twelve have been seen in front of the anal segment. The number seem to vary with the species studied. This of course suggests connection with the anomomeristic trilobites.

The oldest true spiders are found in the Pennsylvanian, and several genera are now known. The head and thorax are fused completely, but the abdomen is distinctly segmented. Some of the Anthracomarti resemble the trilobites more closely than do the Araneæ, as they lack the constriction between the cephalothorax and abdomen. The spiders of the Pennsylvanian have this constriction less perfectly developed than do modern Araneæ, and occupy an intermediate position in this respect. In the Anthracomarti, the pedipalpi are simple, pediform, and all the appendages have very much the appearance of the coxopodites and endopodites of trilobites. Cheliceræ are not known, and pleural lobes are well developed in this group. Anthracomarti have not yet been found in strata older than the Pennsylvanian, but they seem to be to a certain extent intermediate between true spiders and the marine arachnid.

Insecta.

Handlirsch (in several papers, most of which are collected in "Die Fossilen Insekten," 1908) has attempted to show that all the Arthropoda can be derived from the Trilobita, and has advocated the view that the Insecta sprang directly from that group, without the intervention of other tracheate stock. At first sight, this transformation seems almost an impossibility, and the view does not seem to have gained any great headway among entomologists in the fourteen years since it was first promulgated. If an adult trilobite be compared with an adult modern insect, few likenesses will be seen, but when the trilobite is stripped of its specializations and compared with the germ-band of a primitive insect, the theory begins to seem more possible.

Handlirsch really presented very little specific evidence in favor of his theory. In fact, one gets the impression that he has insisted on only two points. Firstly, that the most ancient known insects, the Palæodictyoptera, were amphibious, and their larvæ, which lived in water, were very like the adult. Secondly, that the wings of the Palæodictyoptera probably worked vertically only, and the two main wings were homologous with rudimentary wing-like outgrowths on each segment of the body. These outgrowths have the appearance of, and might have been derived from, the pleural lobes of trilobites.

He figured (1908, p. 1305, fig. 7) a reconstructed larva of a palæodictyopterid as having biramous limbs on each segment, but so far as I can find, this figure is purely schematic, for there seems to be no illustration or description of any such larva in the body of his work.

That the insects arose directly from aquatic animals is of course possible, and Handlirsch's first argument has considerable force. It may, however, be purely a chance that the oldest insects now known to us happen to be an amphibious tribe. The Palæodictyoptera are not yet known to antedate the Pennsylvanian, but there can be no doubt that, insects existed long before that time, and the fact that their remains have not been found is good evidence that the pre-Pennsylvanian insects were not aquatic. Comstock, who has recently investigated the matter, does not believe that the Palæodictyoptera were amphibious (The Wings of Insects, Ithaca, N. Y., 1918, p. 91).

The second argument, that wings arose from the pleural lobes of trilobites, is exceedingly weak. Where most fully set forth (1907, p. 157), he suggests that trilobites may occasionally have left the water, climbed a steep bank or a plant, and then glided back into their native element, taking advantage of the broad flat shape to make a comfortable and gentle descent! This sport apparently became so engaging that the animal tried experiments with flexible wing tips, eventually got the whole of the pleural lobes in a flexible condition, and selected those of the second and third thoracic segments for preservation, while discarding the remainder. The pleural lobes of trilobites are not only too firmly joined to the axial portion of the test to be easily transformed into movable organs, but they are structurally too unlike the veined wings of insects to make the suggestion of this derivation even worthy of consideration.

Tothill (1916) has recently reinvestigated the possible connection between insects, chilopods, and trilobites, and, from the early appearance of the spiracles in the young, came to the conclusion that the insects were derived from terrestrial animals. He suggested that they may have come through the chilopods from the trilobites. The hypothetical ancestor of the insects, as restored by Tothill from the evidence of embryology and comparative anatomy, is an animal more easily derived from the Chilopoda than from the Trilobita. Five pairs of appendages are present on the head, and the trunk is made up of fourteen similar segments, each with a pair of walking limbs and a pair of spiracles.

Only the maxillæ and maxillulæ are represented as biramous. If the ancestor of the Insecta was, as seems possible, tracheate, this fact alone would rule out the trilobites. Among tracheates, the Chilopoda are certainly more closely allied to the Insecta than are any other wingless forms. If the ancestors of the insects were not actually chilopods, they may have been chilopod-like, and there can be little doubt that both groups trace to the same stock.

As to the ancestry of the Chilopoda, it is probable that they had the same origin as the other Arthropoda. Tothill has pointed out that in the embryo of some chilopods there are rudiments of two pairs of antennæ and that the two pairs of maxillæ and the maxillipeds are biramous. This would point rather to the Haplopoda than directly to the trilobites as possible ancestors, and may explain why the former vanish so suddenly from the geological record after their brief appearance in the Middle Cambrian. They may have gone on to the land.

There seem to be no insuperable obstacles to prevent the derivation, indirectly, of the insects from some trilobite with numerous free segments, and small pygidium. The antennules and pleural lobes must be lost, the antennas and trunk limbs modified by loss of exopodites. Wings and tracheæ must be acquired.

Handlirsch places the date of origin of the Insecta rather late, just at the end of the Devonian and during the "Carboniferous." By that time most families of trilobites had died out, so that the possibilities of origin of new stocks were much diminished. If the haplopod-chilopod-insect line is a better approximation to the truth, then the divergence began in the Cambrian.

Chilopoda.

The adult chilopod lacks the antennules, and all of the other appendages, with the exception of the maxillulæ, are uniramous. The walking legs are similar to the endopodites of trilobites, and usually have six or seven segments. The appendages are therefore such as could be derived by modification of those of trilobites by the almost complete loss of the exopodites and shortening of the endopodites of the head. The position of the post-oral appendages, the posterior ones outside those closest the mouth, is perhaps foreshadowed in the arrangement of those of Triarthrus.

The Chilopoda differ from the Hexapoda in developing the antennæ instead of the antennules as tactile organs, but this can not be used with any great effect as an argument that the latter did not arise from the ancestors of the former, since it is entirely possible that in early Palæozoic times the pre-Chilopoda possessed two pairs of antennæ. The first pair are still recognizable in the embryo of certain species.

The oldest chilopods are species described by Scudder (Mem. Boston Soc. Nat. Hist., vol. 4, 1890, p. 417, pl. 38) from the Pennsylvania!! at Mazon Creek, Grundy County, Illinois. Only one of these, _Latzelia primordialis_ Scudder (pl. 38 fig. 3), is at all well preserved. This little animal, less than an inch long, had a depressed body, with a median carina, exceedingly long slender legs, and about nineteen segments. The head is very nearly obliterated.

Diplopoda.

The diplopods, especially the polydesmids with their lateral outgrowths, often have a general appearance somewhat like that of a trilobite, but on closer examination few likenesses are seen. The most striking single feature of the group, the possession by each segment of two pairs of appendages, is not in any way foreshadowed in the trilobites, none of which shows any tendency toward a fusion of pairs of adjacent segments. The antennules are short, antennæ absent, mandibles and maxillulæ much modified, the latter possibly biramous, and the maxillæ absent. The trunk appendages are very similar to those of chilopods, and could readily be derived from the endopodites of trilobites.

The oldest diplopods are found in the Silurian (Ludlow) and Devonian (Lower Old Red) of Scotland, and three species belonging to two genera are known. The oldest is _Archidesmus loganensis_ Peach (1889, p. 123, pl. 4, fig. 4), and the Devonian species are _Archidesmus macnicoli_ Peach and _Kampecaris forfarensis_ Page (Peach 1882, p. 182, pl. 2, fig. 2, 2a, and p. 179, pl. 2, figs. 1-1g). All of these species show lateral expansions like the recent Polydesmidæ, and these of course suggest the pleural lobes of trilobites. All three of the species are simpler than any modern diplopod, for there is only a single pair of appendages on each segment. No _foramina repugnatoria_ were observed, and the eyes of _Kampecaris forfarensis_ as described are singularly like those of a phacopid.

Peach says: "The eye itself is made up of numerous facets which are arranged in oblique rows, the posterior end of each row being inclined downwards and outwards, the facets being so numerous and so close together that the eye simulates a compound one." There is also a protecting ridge which somewhat resembles a palpebral lobe (1882, pl. 7, fig. la). Peach comments on the strength of the test, and from his description it appears that it must have been preserved in the same manner as the test of trilobites. It was punctate, and granules and spines were also present. The presence of the lateral outgrowths in these ancient specimens would seem to indicate that they are primitive features, and may have been inherited. While possibly not homologous with the pleural extensions of trilobites, they may be vestiges of these structures.

The limbs are made up of seven segments which are circular in section and expand at the distal end. The distal one bears one or two minute spines. They are most readily compared with the endopodites of _Isotelus_. The resemblance is, in fact, rather close. The sternal plates are wider and the limbs of opposite sides further apart than in modern diplopods. Except for one pair of antennæ, no cephalic appendages are preserved.

While these specimens do not serve to connect the Diplopoda with the Trilobita, they do show that most of the specializations of the former originated since Lower Devonian times, and lead one to suspect that the derivation from marine ancestors took place very early, perhaps in the Cambrian. If no very close connection with the trilobites is indicated, there is also nothing to show that the diplopods could not have been derived from that group.

Primitive Characteristics of Trilobites.

TRILOBITES THE MOST PRIMITIVE ARTHROPODS.

The Arthropoda, to make the simplest possible definition, are invertebrate animals with segmented body and appendages. The most primitive arthropod would appear to be one composed of exactly similar segments bearing exactly similar appendages, the segments of the appendages themselves all similar to one another. It is highly improbable that this most primitive arthropod imaginable will ever be found, but after a survey of the whole phylum, it appears that the simpler trilobites approximate it most closely.

That the trilobites are primitive is evidenced by the facts that they have been placed at the bottom of the Crustacea by all authors and claimed as the ancestors of that group by some; that Lankester derived the Arachnida from them; and that Handlirsch has considered them the progenitors of the whole arthropodan phylum.

Specializations among the Arthropoda, even among the free-living forms, are so numerous that it would be difficult to make a complete list of them. In discussing the principal groups, I have tried to show that the essential structures can be explained as inherited from the Trilobita, changed in form by explainable modifications, and that new structures, not' present in the Trilobita, are of such a nature that they might be acquired independently in even unrelated groups.

The chief objections to the derivation of the remainder of the Crustacea from the trilobites have been: first, that the trilobites had broad pleural extensions; second, that they had a large pygidium; and lastly, that they had only one pair of tactile antennæ.

It has now been pointed out that many modern Crustacea have pleural extensions, but that they usually bend down at the sides of the body, and also that in the trilobites and more especially in _Marrella_, there was a tendency toward the degeneration of the pleural lobes. A glance at the Mesonacidæ or Paradoxidæ should be convincing proof that in some trilobites the pygidium is reduced to a very small plate.

In regard to the second antennæ standard text-books contain statements which are actually surprising. A compilation shows that the antennæ are entirely uniramous in but a very few suborders, chiefly among the Malacostraca; that they are biramous with both exopodite and endopodite well developed in most Copepoda, Ostracoda, and Branchiopoda; and that the exopodite, although reduced in size, still has a function in some suborders of the Malacostraca. The Crustacea could not possibly be derived from an ancestor with two pairs of uniramous antennæ.

Although I have defended the trilobites, perhaps with some warmth, from the imputation that they were Arachnida, my argument does not apply in the opposite direction, and I believe Lankester was right in deriving the Arachnida from them. If the number of appendages in front of the mouth is fundamental, then the trilobites were generalized, primitive, and capable of giving rise to both' Crustacea and Arachnida. As shown on a previous page (p. 119), the "connecting links" so far found tend to disprove rather than to prove the thesis, but the present finds should be looked upon as only the harbingers of the greater ones which are sure to come.

LIMBS OF TRILOBITES PRIMITIVE.

The general presence, in an adult or larva, of some sort of biramous limbs throughout the whole class Crustacea has led most zoologists to expect such a limb in the most primitive crustaceans, and apparently the appendage of the trilobite satisfies the expectation. It is well, perhaps, as a test, to consider whether by modification this limb could produce the various types of limbs seen in other members of the class. In the first place, it is necessary to have clearly in mind the peculiarities of the appendage to be discussed.

It should first of all be remembered that the limb is articulated with the dorsal skeleton in a manner which is very peculiar for a crustacean. The coxopodite swings on a sort of ball-and-socket joint, and at the outer end both the exopodite and the basipodite articulate with it. Since the exopodite articulates with the basipodite as well as with the coxopodite, the two branches are closely connected with one another and there is little individual freedom of movement. This is, of course, a necessary consequence of their articulation with a segment which is itself too freely movable to provide a solid base for attachment of muscles. The relation of the appendifer, coxopodite, and two rami is here shown diagrammatically (fig. 33), the exopodite branching off from the proximal end of the basipodite at the junction with the coxopodite.

In all trilobites the endopodite consists of six segments, and the coxopodite of a single segment the inner end of which is prolonged as an endobase. There does not seem to be any variation from this plan in the subclass, although individual segments are variously modified. The exopodites are more variable, but all consist of a flattened shaft with setæ on one margin. No other organs such as accessory gills, swimming plates, or brood pouches have yet been found attached to the appendages, the evidence for the existence of the various epipodites and exites described by Walcott being unsatisfactory (see p. 23).

In the Ostracoda the appendages are highly variable, but it is easily seen that they are modifications of a limb which is fundamentally biramous. In most species, both exopodite and endopodite suffer reduction. The exopodite springs from the basipodite and that segment is closely joined to the coxopodite, producing a protopodite. In some cases the original segments of the endopodites fuse to form a stiff rod. While highly diversified, these appendages are very trilobite-like, and some Ostracoda even have biramous antennæ.

The non-parasitic Copepoda have limbs exceedingly like those of trilobites. Many of them are biramous, the endopodites sometimes retaining the primitive six segments. Coxopodite and basipodite are generally united, and endopodite and exopodite variously modified. Like some of the Ostracoda, the more primitive Copepoda have biramous antennæ.

As would be expected, the appendages of the Cirripedia are much modified, although those of the nauplius are typical. The thoracic appendages of many are biramous, but both branches are multisegmented.

In the modern Malacostraca the ground plan of the appendages is biramous, but in most orders they are much modified. In many, however, the appendages of some part of the body are biramous, and in many the endopodites show the typical six segments. From the coxopodites arise epipodites, some of which assist in swimming, and some in respiration. Because of the many instances in which such extra growths arise, and because of the form of the appendages of the Branchiopoda, it has been suggested that the primitive crustacean leg must have been more complex than that of the trilobite. In looking over the Malacostraca, however, one is struck by the fact that epipodites generally arise where the exopodites have become aborted or are poorly developed, and seem largely to replace them. The coxopodite and basipodite are usually fused to form a protopodite, and a third segment is sometimes present in the proximal part of the appendage.

In the Branchiopoda are found the most complex crustacean limbs, and the ones most difficult to homologize with those of trilobites. In recent years, Lankester's homologies of the parts of the limbs of _Apus_ with those of the Malacostraca have been quite generally accepted, and the appendages of the former considered primitive. Now that it is known that the Branchiopoda of the Middle Cambrian (_Burgessia_ _et at._) had simple trilobite-like appendages, it becomes necessary to exactly reverse the opinion in this matter. The same homologies stand, but the thoracic limbs of _Apus_ must be looked upon as highly specialized instead of primitive.

Lankester (Jour. Micros. Sci., vol. 21, 1881) pointed out that the axial part of the thoracic limb of _Apus_ (fig. 34) is homologous with the protopodite in the higher Crustacea, that the two terminal endites corresponded to the exopodite and endopodite, and that the other endites and exites were outgrowths from the protopodite analogous to the epipodites of Malacostraca. There seems to be no objection to retaining this interpretation, but with the meaning that both endopodite and exopodite are much reduced, and their functions transferred to numerous outgrowths of the protopodite. One of the endites grows inward to form an endobase, the whole limb showing an attempt to return to the ancestral condition of the trilobite. The limbs of some other branchiopods are not so easy to understand, but students of the Crustacea seem to have worked out a fairly satisfactory comparison between them and _Apus_.

The discovery that the ancestral Branchiopoda had simple biramous appendages instead of the rather complex phyllopodan type is another case in which the theory of "recapitulation" has proved to hold. It had already been observed that in ontogeny the biramous limb preceded the phyllopodan, but so strong has been the belief in the primitive character of the Apodidæ that the obvious suggestion has been ignored. Even in such highly specialized Malacostraca as the hermit crabs the development of certain of the limbs illustrates the change from the schizopodal to the phyllopodan type, and Thompson (Proc. Boston Soc. Nat. Hist., vol. 31, 1903, pl. 5, fig. 12) has published an especially good series of drawings showing the first maxilliped. In the first to fourth zoeæ the limb is biramous but in the glaucothoe a pair of broad processes grow out from the protopodite, while the exopodite and particularly the endopodite become greatly reduced. In the adult the endopodite is a mere vestige, while the flat outgrowths from the protopodite have become very large and bear setæ.

_Summary._

The limbs of most Crustacea are readily explained as modifications of a simple biramous type. These modifications usually take the form of reduction by the loss or fusion of segments and quite generally either the entire endopodite or exopodite is lacking. Modification by addition frequently occurs in the growth of epipodites, "endites," and "exites" from the coxopodite, basipodite, or both. A protopodite is generally formed by the fusion of coxopodite and basipodite, accompanied by a transference of the proximal end of the exopodite to the distal end of the basipodite. A new segment, not known in the trilobites (precoxal), is sometimes added at the inner end.

Among modern Crustacea, the anterior cephalic appendages and thoracic appendages of the Copepoda and the thoracic appendages of certain Malacostraca, Syncarida especially, are most nearly like those of the trilobite. The exact homology, segment for segment, between the walking legs of the trilobite and those of many of the Malacostraca, even the Decapoda, is a striking instance of retention of primitive characteristics in a specialized group, comparable to the retention of primitive appendages in man.

NUMBER OF SEGMENTS IN THE TRUNK.

Various attempts have been made to show that despite the great variability, trilobites do show a tendency toward a definite number of segments in the body.

Emmrich (1839), noting that those trilobites which had a long thorax usually had a short pygidium, and that the reverse also held true, formulated the law that the number of segments in the trunk was constant (20 + 1) Very numerous exceptions to this law were, however, soon discovered, and while the condition of those with less than twenty-one segments was easily explained, the increasing number of those with more than twenty-one soon brought the idea into total disrepute.

Quenstedt (1837) had considered the number of segments of at least specific importance, and both he and Burmeister (1843) considered that the number of segments in the thorax must be the same for all members of a genus. As first shown by Barrande (1852. p. 191 et seq.), there are very many genera in which there is considerable variation in the number of thoracic segments, and a few examples can be cited in which there is variation within a species, or at least in very closely related species.

Carpenter (1903, p. 333) has tabulated the number of trunk segments of such trilobites as were listed by Zittel in 1887 and finds a steady increase throughout the Palæozoic. His table, which follows, is, however, based upon very few genera.

Period No. of Genera Average No. of body-segments =============================================== Cambrian 12 17.66 Ordovician 23 18.58 Silurian 16 19.34 Devonian 10 20.70 Carboniferous 2 20.75

Due chiefly to the efforts of Walcott, an increasingly large number of Cambrian genera are now represented by entire specimens, and since these most ancient genera are of greatest importance, a few comments on them may be offered.

The total number of segments can be fairly accurately determined in at least nineteen genera of trilobites from the Lower Cambrian. These include eight genera of the Mesonacidæ (_Olenellus_ was excluded) and _Eodiscus_, _Goniodiscus_, _Protypus_, _Bathynotus_, _Atops_, _Olenopsis_, _Crepicephalus_, _Vanuxemella_, _Corynexochus_, _Bathyuriscus_, and _Poliella_. The extremes of range in total segments of the trunk is seen in _Eodiscus_ (9) and _Pædeumias_ (45+), and these same genera show the extremes in the number of thoracic segments, there being 3 in the one and 44+ in the other. _Pædeumias_ probably shows the greatest variation of any one genus of trilobites, various species showing from 19 to 44+ thoracic segments. The average for the nineteen genera is 13.9 segments in the thorax, 3.7 segments in the pygidium, or a total average of 17.6 segments in the trunk. _Crepicephalus_ with 12-14 segments in the thorax and 4-6 in the pygidium, and _Protypus_, with 13 in the thorax and 4-6 in the pygidium, are the only genera which approach the average. All of the Mesonacidæ, except one, _Olenelloides_, have far more thoracic and fewer pygidial segments than the average, while the reverse is true of the Eodiscidæ, _Vanuxemella_, _Corynexochus_, _Bathyuriscus_, and Poliella.

The eight genera of the Mesonacidæ, _Nevadia_, _Mesonacis_, _Elliptocephala_, _Callavia_, _Holmia_, _Wanneria_, _Pædeumias_, and _Olenelloides_, have an average of 20.25 segments in the thorax and 1.5 in the pygidium, a total of 21.75. If, however, the curious little _Olenelloides_ be omitted, the average for the thorax rises to 22.14 and the total to 23.84. _Olenelloides_ is, in fact, very probably the young of an _Olenellus_. Specimens are only 4.5 to 11 mm. long, and occur in the same strata with _Olenellus_ (see Beecher 1897 A, p. 191).

Thirty-three genera from the Middle Cambrian afford data as to the number of segments, the Agnostidæ being excluded. The extreme of variation there is smaller than in the Lower Cambrian. The number of thoracic segments varies from 2 in Pagetia to 25 in _Acrocephalites_, and these same genera show the greatest range in total number of trunk segments, 8 and 29 respectively.

The average of thoracic segments for the entire thirty-three genera is 10.5, of pygidial segments 5.9, a total average of 16.4. It will be noted that the thorax shows on the average less and the pygidium more segments than in the Lower Cambrian. If the Agnostidæ could be included, this result would doubtless be still more striking. Of the genera considered, _Asaphiscus_ with 7-11 thoracic and 5-8 pygidial segments, _Blainia_ with 9 thoracic and 6-11 pygidial, _Zacanthoides_ with 9 thoracic and 5 pygidial, and _Anomocare_ with 11 thoracic and 7-8 pygidial segments came nearest to the average. Only a few departed widely from it. The genera tabulated were _Acrocephalites_, _Alokistocare_, _Crepicephalus_, _Karlia_, _Hamburgia_, _Corynexochus_, _Bathyuriscus_, Poliella, _Agraulos_, _Dolichometopus_, _Ogygopsis_, _Orria_, _Asaphiscus_, _Neolenus_, _Burlingia_, _Blainia_, _Blountia_, _Marjumia_, _Pagetia_, _Eodiscus_, _Goniodiscus_, _Albertella_, _Oryctocara_, _Zacanthoides_, _Anomocare_, _Anomocarella_, _Coosia_, _Conocoryphe_, _Ctenocephalus_, _Paradoxides_, _Ptychoparia_, _Sao_, and _Ellipsocephalus_.

Enough genera of Upper Cambrian trilobites are not known from entire specimens to furnish satisfactory data. Excluding from the list the Proparia recently described by Walcott, the average total trunk segments in ten genera is 18, but as most of the genera are Olenidæ or olenid-like, not much weight can be attached to these figures.

For the Cambrian as a whole, the average for sixty-two genera is between 17 and 18 trunk segments, which is surprisingly like the result obtained by Carpenter from only twelve genera, and tends to indicate that it must be somewhere near the real average. If the 5 or 6 segments of the head be added, it appears that the "average" number of segments is very close to the malacostracan number 21. Genera with 16 to 18 trunk segments are Callavia, _Protypus_, _Bathynotus_, _Crepicephalus_, _Bathyuriscus_, _Ogygopsis_, _Burlingia_, _Orria_, _Asaphiscus_, _Blainia_, _Zacanthoides_, _Neolenus_, _Anomocare_, _Conocoryphe_, _Saukia_, _Olenus_, and _Eurycare_.

The order Proparia originated in the Cambrian, and Walcott has described four genera, one from the Middle, and three from the Upper. The number of segments in these genera is of interest. _Burlingia_, the oldest, has 14 segments in the thorax and 1 in the pygidium. Of the three genera in the Upper Cambrian, _Norwoodia_ has 8-9 segments in the thorax and 3-4 in the pygidium; _Millardia_ 23 in thorax and 3-4 in pygidium; and _Menomonia_ 42 in thorax and 3-4 in pygidium. It is of considerable interest and importance to note that the very elongate ones are not from the Middle but from the Upper Cambrian.

Forty genera of Ordovician trilobites known from entire specimens were tabulated, and it was found that the range in the number of segments in the thorax and pygidium was surprisingly large. _Agnostus_, which was not included in the table, has the fewest, and _Eoharpes_, with 29, the most. While the range in number of segments in the thorax is 2 to 29, the range of the number in the pygidium, 2 to 26, is almost as great. A species of _Dionide_ has 26 in the pygidium, while _Remopleurides_ and _Glaphurus_ have evidence of only 2. The average number of segments in the thorax for the forty genera was 10.15, in the pygidium 8.81, and the average number for the trunk 19.

Genera with just 19 segments in the trunk appear to be rare in the Ordovician, a species of _Ampyx_ being the only one I have happened to notice. _Calymene_, _Tretaspis_, _Triarthrus_, _Asaphus_, _Ogygites_, and _Goldius_ come with the range of 18 to 20. _Goldius_, with 10 segments in the thorax and (apparently) 8 in the pygidium, comes nearest to the averages for these two parts of the trunk. _Goldius_, _Amphilichas_, _Bumastus_, _Acidaspis_, _Actinopeltis_, and _Sphærexochus_ are among the genera having 10 segments in the thorax, and there are many genera which have only one or two segments more or less than 10.

In most Ordovician genera, thirty-five out of the forty tabulated, the number of segments in the thorax is fixed, and the variation is in any case small. In four of the five genera where it was not fixed, there was a variation of only one segment, and the greatest variation was in _Pliomerops_, where the number is from 15 to 19. This of course indicates that the number of segments in the thorax tends to become fixed in Ordovician time. The variation in the number of segments in the pygidium is, however, considerable. It is difficult in many cases to tell how many segments are actually present in this shield, as it is more or less smooth in a considerable number of genera. Extreme cases of variation within a genus are found in _Encrinurus_, species of which have from 7 to 22 segments in the pygidium, _Cybeloides_ with 10 to 20, and _Dionide_ with 10 to 26. As the number in the thorax became settled, the number in the pygidium became more unstable, so that not even in the Ordovician can the total number of segments in the trunk be said to show any tendency to become fixed.

The genera used in this tabulation were: _Eoharpes_, _Cryptolithus_, _Tretaspis_, _Trinucleus_, _Dionide_, _Raphiophorus_, _Ampyx_, _Endymionia_, _Anisonotus_, _Triarthrus_, _Remopleurides_, _Bathyurus_, _Bathyurellus_, _Ogygiocaris_, _Asaphus_, _Ogygites_, _Isotelus_, _Goldius_, _Cyclopyge_, _Amphilichas_, _Odontopleura_, _Acidaspis_, _Glaphurus_, _Encrinurus_, _Cybele_, _Cybeloides_, _Ectenonotus_, _Calymene_, _Ceraurus_, _Pliomera_, _Pliomerops_, _Pterygometopus_, _Chasmops_, _Eccoptochile_, _Actinopeltis_, _Sphærexochus_, _Placoparia_, _Pilekia_, _Selenopeltis_, and _Calocalymene_.

Only sixteen genera of Devonian trilobites were available for tabulation, and it is not always possible to ascertain the exact number of segments in the pygidium, although genera with smooth caudal shields had nearly all disappeared. The number of segments in the thorax had become pretty well fixed by the beginning of the Devonian, _Cyphaspis_ with a range of from 10 to 17 furnishing the only notable exception. The range for the sixteen genera is from 8 to 17, the average 11, the number exhibited by the Phacopidæ which form so large a part of the trilobites of the Devonian. The greater part of the species have large pygidia, and while the range is from 3 to 23, the average is 11.2. _Probolium_, with 11 in the thorax and 11-13 in the pygidium, and _Phacops_, with 11 in the thorax and 9-12 in the pygidium, approach very closely to the "average" trilobite, and various species of other genera of the Phacopidæ have the same number of segments as the norm. In every genus, however, the number of segments in the pygidium is variable, the greatest variation being in _Dalmanites_, with a range of from 9 to 23. The number of segments in the pygidium was therefore not fixed and was on the average higher than in earlier periods.

The genera used in the tabulation were: _Calymene_, _Dipleura_, _Goldius_, _Proëtus_, _Cyphaspis_, _Acidaspis_, _Phacops_, _Hausmania_, _Coronura_, _Odontochile_, _Pleuracanthus_, _Calmonia_, _Pennaia_, _Dalmanites_, _Probolium_, and _Cordania_.

The trilobites of the late Palæozoic (Mississippian to Permian) belong, with two possible exceptions, to the Pröetidæ, and only three genera, _Proëtus_, _Phillipsia_, and _Griffithides_, appear to be known from all the parts. I am, however, assuming that both _Brachymetopus_ and _Anisopyge_ have 9 segments in the thorax, and so have tabulated five genera. The range in the number of segments in the pygidium is large, from 10 in some species of _Proëtus_ to 30 in _Anisopyge_, and the average, 17.3, is high, as is the average for total number in the trunk, 26.3. _Anisopyge_, a late Permian trilobite described by Girty from Texas, is perhaps the last survivor of the group. It seems to have had 39 segments in the trunk, making it, next to the Cambrian _Pædeumias_ and _Menomonia_, the most numerously segmented of all the trilobites.

The above data may be summarized in the following table:

Period No. of Av. No. of Av. No. of Av. No. genera segments in segments in of trunk thorax pygidium segments ========================================================== Lower Cambrian 19 13.9 3.7 17.6 Middle Cambrian 33 10.5 5.9 16.4 Entire Cambrian 62 ... ... 17-19 Ordovician 40 10.15 8.81 18.96 Devonian 16 11 11.2 22.2 Late Palæozoic 5 9 17.3 26.3

This table confirms that made up by Carpenter, and shows even more strikingly the progressive increase in the average number of segments in the trunk throughout the Palæozoic.

While the two trilobites with the greatest number of segments are Cambrian, yet on the average, the last of the trilobites had the more numerously segmented bodies. The multisegmented trilobites are:

Period Genus Av. No. of Av. No. of Av. No. segments in segments in of trunk thorax pygidium segments ================================================================ Lower Cambrian _Pædeumias_ 44+ 1 45+ Upper Cambrian _Menomonia_ 42 4 46 _Ectenonotus_ 12 22 34 Ordovician _Encrinurus_ 11 22 33 _Dionide_ 6 26 32 Silurian _Harpes_ 29 3 32 Devonian _Coronura_ 11 23 34 _Dalmanites_ 11 23 34 Permian _Anisopyge_ 7+(9?) 30 39?

_Anisopyge_, the last of the trilobites, stands third on the list of those having great numbers of segments, and in each period there are a few which have considerably more than the average number. It may be of some significance that of these nine genera only _Pædeumias_ and _Anisopyge_ belong to the Opisthoparia, the great central group, and that five are members of the Proparia, the latest and most specialized order.

FORM OF THE SIMPLEST PROTASPIS.

It would naturally be expected that the young of the Cambrian trilobites should be more primitive than the young of species from later formations, and Beecher (1895 C) has shown that this is the case. He had reference, however, chiefly to the eyes, free cheeks, and spines, and by comparison of ontogeny and phylogeny, demonstrated the greater simplicity of the protaspis which lacked these organs. It remains to inquire which among the other characteristics are most fundamental.

Among the trilobites of the Lower Cambrian, no very young have been seen except of Mesonacidæ. Of these, the ontogeny of _Elliptocephala asaphoides_ Emmons is best known, thanks to Ford, Walcott, and Beecher, but, as the last-named has pointed out, the actual protaspis or earliest shield has not yet been found. The youngest specimen is the one roughly figured by Beecher (1895 C, p. 175, fig. 6). It lacks the pygidium, but if completed by a line which is the counterpart of the outline of the cephalon, it would have been 0.766 mm. long. The pygidium would have been 0.183 mm. long, or 23 per cent of the whole length. The axial lobe was narrow, of uniform width along the cephalon, showed a neck-ring and four indistinct annulations, but did not reach quite to the anterior end, there being a margin in front of the glabella about 0.1 mm. wide. The greatest width of the cephalon was 0.66 mm., and of the glabella 0.233 mm., or practically 35 per cent of the total width. Other young _Elliptocephala_ up to a length of 1 mm., and young _Pædeumias_, _Mesonacis_, and _Holmia_ (see Kiær, Videnskaps, Skrifter, 1 Mat.-Naturv. Klasse, 1917, No. 10) show about the same characteristics, but all these have large compound eyes on the dorsal surface and specimens in still younger stages are expected. It may be pointed out, however, that in these specimens the pygidium is proportionately larger than in the adult. Walcott cites one adult 126 mm. long in which the pygidium is 6 mm. long, or between 4 and 5 per cent of the total length, while in the incomplete specimen described above, it was apparently 23 per cent. In a specimen 1 mm. long figured by Walcott, the pygidium is 0.15 mm. long, or 15 per cent of the whole length.

The development of several species of trilobites from the Middle Cambrian is known. Barrande (1852) described the protaspis of _Sao hirsuta_, _Peronopsis integer_, _Phalacroma bibullatum_, _P. nudum_, and _Condylopyge rex_. Broegger figured that of a _Liostracus_ (Geol. For. Förhandl., 1875, pl. 25, figs. 1-3) and Lindstroem (1901, p. 21) has reproduced the same. Matthew (Trans. Roy. Soc. Canada, vol. 5, 1888, pl. 4, pls. 1, 2) has described the protaspis of a _Liostracus_, _Ptychoparia linnarssoni_ Broegger, and _Solenopleura robbi_ Hartt. Beecher (1895 C, pl. 8) has figured the protaspis of _Ptychoparia kingi_ Meek, and the writer that of a Paradoxides (Bull. Mus. Comp. Zool., vol. 58, No. 4, 1914, pl. i).

_Sao_, _Liostracus_, _Ptychoparia_, and _Solenopleura_ all have the same sort of protaspis. In all, the axial lobe reaches the anterior margin and is somewhat expanded at that end; in all, the glabella shows but slight trace of segmentation; and in all, the pygidium occupies from one fifth to one fourth the total length. There is considerable variation in the width of the axial lobe. It is narrowest in _Ptychoparia_, where in the middle it is only 14 per cent of the whole width, and widest in _Solenopleura_, where it is 28 per cent. In _Ptychoparia_ the pygidium of the protaspis occupies from 18 to 22 per cent of the whole length. In the adult it occupies 10 to 12 per cent. In _Solenopleura_ it makes up about 26 per cent of the protaspis, and in the adult about 8 per cent.

In the youngest stages of all these trilobites, the pygidium is incompletely separated from the cephalon. The first sign of segmentation is a transverse crack which begins to separate the cephalon and pygidium, and by the time this has extended across the full width the neck segment has become rather well defined. In this stage the animal is prepared to swim by means of the pygidium, and first becomes active. The coincident development of the free pygidium and the neck-ring strongly suggests that the dorsal longitudinal muscles are attached beneath the neck-fur row.

The single protaspis of _Paradoxides_ now known, while only 1 mm. long, is not in the youngest stage of development. It is like the protaspis of _Olenellus_ in having large eyes on the dorsal surface and a narrow brim in front of the glabella. The glabella is narrower than in the adult.

The initial test of no agnostid has probably as yet been seen, as all the young now known show the cephalon and pygidium distinctly separated. _Phalacroma bibullatum_ and _P. nudum_ are both practically smooth and isopygous when 1.5 mm. long. _P. bibullatum_ shows no axial lobe at this stage, but a wide glabella and median tubercle develop later, and when the glabella first appears, it extends to the anterior margin. In _Peronopsis integer_ and _Condylopyge rex_, the axial lobe is outlined on each of the equal shields in specimens about 1 mm. long, but is without furrows and reaches neither anterior nor posterior margin.

From the foregoing brief description it appears that the pygidium of the protaspis varies in different groups from as little as 15 per cent of the total length in the Mesonacidæ to as much as 50 per cent in the Agnostidæ; that the axial lobe varies from as little as 14 per cent of the total width in one _Ptychoparia_ to as much as 50 per cent in _Phalacroma nudum_; that the glabella reaches the anterior margin in the Olenidæ, Solenopleuridæ, and _Phalacroma bibullatum_, while there is a brim in front of it in the Olenellidæ, Paradoxidæ, and three of the species of the Agnostidæ. The decision as to which of these conditions are primitive may be settled quite satisfactorily by study of the ontogeny of the various species.

ORIGIN OF THE PYGIDIUM.

Taking first the pygidium, it has already been pointed out that in each case the pygidium of the adult is proportionally considerably smaller than the pygidium of the protaspis. The stages in the growth of the pygidium are better known in Sao hirsuta than in any other trilobite, and a review of Barrande's description will be advantageous.

Barrande recognized twenty stages in the development of this species, but there was evidently a still simpler protaspis in his hands than the smallest he figured, for he says, after describing the specimen in the first stage: "We possess one specimen on which the head extends from one border to the other of the disk, but as this individual is unique we have not thought it sufficient to establish a separate stage." This specimen is important as indicating a stage in which there was not even a suggestion of division between cephalon and pygidium.

In the first stage described by Barrande, the form is circular, the length is about 0.66 mm., and the glabella is narrow with parallel sides and no indications of lateral furrows. The neck segment is indicated by a slight prominence on the axial lobe, and back of it a constriction divides the axial lobe of the pygidium into two nodes, but does not cross the pleural lobes. The position of the nuchal segment permits a measurement of the part which is to form the pygidium, and shows that that shield made up 30 per cent of the entire length.

In the second stage, when the test is 0.75 mm. long, the cephalon and pygidium become distinctly separated, and the latter shield shows three annulations on the axial and two pairs of ribs on the pleural lobes. It now occupies 33-1/3 per cent of the total length.

In the third stage, when the total length is about 1 mm., the pygidium has continued to grow. It now shows five annulations on the axial lobe, and is 46 per cent of the total length.

In the fourth stage, two segments of the axial lobe have been set free from the front of the pygidium. The length is now 1.5 mm. and the pygidium makes up 32 per cent of the whole. From this time the pygidium continues to decrease in size in proportion to the total length, as shown in the following table.

Stage Length in Percentage Segments in Segments in mm. of pygidium thorax pygidium ======================================================== 1 0.66 30 0 2 2 0.75 33-1/3 0 3 3 1.00 46 0 5 4 1.50 32 2 5-6 5 1.50 25 3 4 6 1.75 23 4 4 7 1.80 21 5 3 8 2.00 17 6 3 9 2.50 13 7 3 10 3.00 12 8 3 11 3.50 11 9 3-4 12 4.00 11 10 3-4 13 5.00 10 11 3 14 5.50 9 12 2-4 15 6.00 8 13 3-4 16 6.50 8 14 3 17 7.00 7 15 3 18 7.50 7 16 3 19 7.50 6 17 2 20 10.25 6 17 2

This table shows the rapid increase in the length of the pygidium till the time when the thorax began to be freed, the very rapid decrease during the earlier part of its formation until six segments had been set free, and then a more gradual decrease until the entire seventeen segments had been acquired, after which time the relative length remained constant. From an initial proportion of 30 per cent, it rose to nearly one half the whole length, and then dwindled to a mere 6 per cent, showing conclusively that the thorax grew at the expense of the pygidium.

If this conclusion can be sustained by other trilobites, it indicates that the large pygidium is a more primitive characteristic of a protaspis than is a small one. I have already shown that the pygidium is proportionately larger in the protaspis in the Mesonacidæ, Solenopleuridæ, and Olenidæ, and a glance at Barrande's figures of _"Hydrocephalus" carens_ and _"H." saturnoides_, both young of _Paradoxides_ will show that the same process of development goes on in that genus as in _Sao_. There is first an enlargement of the pygidium to a maximum, a rise from 20 per cent to 33 per cent in the case of _H. carens_ and then, with the introduction of thoracic segments, a very rapid falling off. All of these are, however, trilobites with small pygidia, and it has been a sort of axiom among palæontologists that large pygidia were made up of a number of coalesced segments. While not definitely so stated, it has generally been taken to mean the joining together of segments once free. The asaphid, for instance, has been thought of as descended from some trilobite with rich segmentation, and a body-form like that of a _Mesonacis_ or _Paradoxides_.

The appeal to the ontogeny does not give as full an answer to this question as could be wished, for the complete life-history of no trilobite with a large pygidium is yet known. While the answer is not complete, enough can be gained from the study of the ontogeny of _Dalmanites_ and _Cyclopyge_ to show that in these genera also the thorax grows by the breaking down of the pygidium and that no segment is ever added from the thorax to the pygidium. The case of _Dalmanites socialis_ as described by Barrande (1852, p. 552, pl. 26) will be taken up first, as the more complete. The youngest specimen of this species yet found is 0.75 mm. long, the pygidium is distinctly separated from the cephalon, and makes up 25 per cent of the length. This is probably not the form of the shell as it leaves the egg. At this stage there are two segments in the pygidium, but they increase to four when the test is 1 mm. long. The cephalon has also increased in length, however, so that the proportional length is the same. The subjoined table, which is that compiled by Barrande with the proportional length of the pygidium added, is not as complete as could be desired, but affords a very interesting history of the growth of the caudal shield. The maximum proportional length is reached before the introduction of thoracic segments, and during the appearance of the first five segments the size of the pygidium drops from 25 to 15 per cent. Several stages are missing at the critical time between stages 8 and 9 when the pygidium had added three segments to itself and has supplied only one to the thorax. This would appear to have been a sort of resting or recuperative stage for the pygidium, for it increased its own length to 20 per cent, but from this stage up to stage 12 it continued to give up segments to the thorax and lose in length itself. After stage 12, when the specimens were 8 mm. long, no more thoracic segments were added, but new ones were introduced into the pygidium, until it reached a size equal to one fifth the entire length, as compared with one fourth in the protaspis.

Stage Length Percentage Segments in Segments in in mm. of pygidium thorax pygidium ==================================================== 1 0.75 25 0 2 2 0.75 25 0 3 3 1.00 25 0 4 4 1.00 22 1 3 5 1.25 20 2 3 6 1.25 18 3 3 7 1.60 15 4 3 8 1.60 15 5 3 9 3.00 20 6 6 10 3.50 20 7 6 11 8.00 18 9 7 12 8.00 16 11 5 13 12.00 16 11 7 14 19.00 18 11 9 15 95.00 20 11 11

Since the above was written, Troedsson (1918, p. 57) has described the development of _Dalmanites eucentrus_, a species found in the Brachiopod shales (Upper Ordovician) of southern Sweden. This species follows a course similar to that of _D. socialis_, so that the full series of stages need not be described. The pygidium is, however, of especial interest, for there is a stage in which it shows two more segments than in the adult. Troedsson figures a pygidium 1.28 mm. long which has eight pairs of pleural ribs, while the adult has only six pairs. The ends of all these ribs are free spines, and were the development not known one would say that this was a case of incipient fusion, while as a matter of fact, it is incipient freedom.

A further interest attaches to this case, because of the close relationship between _D. eucentrus_ and _D. mucronatus_. The latter species appears first in the _Staurocephalus_ beds which underlie the Brachiopod shales, so that in its first appearance it is somewhat the older. The pygidium of the adult _D. mucronatus_ is larger than that of _D. eucentrus_, having eight pairs of pleural ribs, the same number as in the young of the latter. In short, _D. eucentrus_ is probably descended from _D. mucronatus_, and in its youth passes through a stage in which it has a large pygidium like that species. Once more it appears that the small pygidium is more specialized than the large one.

The full ontogeny of _Cyclopyge_ is not known, but young specimens show conclusively that segments are not transferred from the thorax to the pygidium, but that the opposite occurs. As shown by Barrande (1852) and corroborated by specimens in the Museum of Comparative Zoology, the process is as follows: The third segment of the adult of this species, that is, the fourth from the pygidium, bears a pair of conspicuous cavities on the axial portion. In a young specimen, 7 mm. long, the second segment bears these cavities, but as the thorax has only four segments, this segment is also the second instead of the fourth ahead of the pygidium. The pygidium itself, instead of being entirely smooth, as in the adult state, is smooth on the posterior half, but on the anterior portion has two well formed but still connected segments, the anterior one being more perfect than the other. These are evidently the two missing segments of the thorax, and instead of being in the process of being incorporated in the pygidium, they are in fact about to be cast off from it to become free thoracic segments. In other words, the thorax grows through the degeneration of the pygidium. That the thorax grows at actual expense to the pygidium is shown by the proportions of this specimen. In an adult of this species the pygidium, thorax, and cephalon are to each other as 9:11:13. In the young specimen they are as 10:6:12, the pygidium being longer in proportion both to the thorax and to the cephalon than it would be in the adult.

This conception of the breaking down of the pygidium to form the thorax will be very helpful in explaining many things which have hitherto seemed anomalous. For instance, it indicates that the Agnostidæ, whose subequal shields in early stages have been a puzzle, are really primitive forms whose pygidia do not degenerate; likewise the Eodiscidæ, which, however, show within the family a tendency to free some of the segments. The annelidan Mesonacidæ may not be so primitive after all, and their specialized cephala may be more truly indicative of their status than has previously been supposed.

The facts of ontogeny of trilobites with both small and large pygidia do show that there is a reduction of the relative size of the caudal shield during the growth-stages, and therefore that the large pygidium in the protaspis is probably primitive. The same study also shows that the large pygidium is made up of "coalesced segments" only to the extent that they are potentially free, and not in the sense of fused segments.

WIDTH OF THE AXIAL LOBE.

That the narrow type of axial lobe is more primitive than the wide one has already been demonstrated by the ontogeny of various species, and space need not be taken here to discuss the question. Most Cambrian trilobites have narrow axial lobes even in the adult so that their development does not bring this out very strikingly, though it can be seen in Sao, Ptychoparia, etc., but in Ordovician trilobites such as Triarthrus and especially Isotelus, it is a conspicuous feature.

PRESENCE OR ABSENCE OF A "BRIM."

That the extension of the glabella to the front of the cephalon is a primitive feature is well shown by the development of Sao (Barrande, 1852, pl. 7), Ptychoparia (Beecher, 1895 C, pl. 8), and Paradoxides (Raymond, Bull. Mus. Comp. Zool., vol. 57, 1914), although in the last genus the protaspis has a very narrow brim, the larva during the stages of introduction of new segments a fairly wide one, and most adults a narrow one.

The brim of Sao seems to be formed partly by new growth and partly at the expense of the frontal lobe, for that lobe is proportionately shorter in the adult than in the protaspis. In _Cryptolithus_ and probably in _Harpes_, _Harpides_, etc., the brim is quite obviously new growth and has nothing to do with the vital organs. Its presence or absence may not have any great significance, but when the glabella extends to the frontal margin, it certainly suggests a more anterior position of certain organs. In _Sao_, the only trilobite in which anything is known of the position of the hypostoma in the young, the posterior end is considerably further forward in a specimen a. 5 mm. long than in one 4 mm. long, thus indicating a backward movement of the mouth during growth, comparable to the backward movement of the eyes.

SEGMENTATION OF THE GLABELLA.

The very smallest specimens of _Sao_ show a simple, unsegmented axial lobe, and the same simplicity has been noted in the young of other genera. Beecher considered this as due to imperfect preservation of the exceedingly small shells, which practically always occur as moulds or casts in soft shale. There is, however, a very general increase in the strength of glabellar segmentation in the early part of the ontogeny of all trilobites whose life history is known, and in some genera, like the Agnostidæ, there is no question of the comparatively late acquisition of glabellar furrows. Even in _Paradoxides_, the furrows appear late in the ontogeny.

_Summary._

If absence of eyes on the dorsal surface be primitive, as Beecher has shown, and if the large pygidium, narrow axial lobe, and long unsegmented glabella be primitive, then the known protaspis of the Mesonacidæ and Paradoxidæ is not primitive, that of the Olenidæ is very primitive, and that of the Agnostidæ is primitive except that in one group the axial lobe, when it appears, is rather wide, and in the other a brim is present.

Subsequent development from the simple unsegmented protaspis would appear to show, first, an adaptation to swimming by the use of the pygidium; next, the invagination of the appendifers as shown in the segmentation of the axial lobe indicates the functioning of the appendages as swimming legs; then with the introduction of thoracic segments the assumption of a bottom-crawling habit is indicated. Some trilobites were fully adapted for bottom life, and the pygidium became reduced to a mere vestige in the production of a worm-like body. Other trilobites retained their swimming habits, coupled with the crawling mode of life, and kept or even increased (_Isotelus_) the large pygidium.

The Simplest Trilobite.

In the discussion above I have placed great emphasis on the large size of the primitive pygidium, because, although there is nothing new in the idea, its significance seems to have been overlooked.

If the large pygidium is primitive, then multisegmentation in trilobites can not be primitive but is the result of adaptation to a crawling life. It is annelid-like, but is not in itself to be relied upon as showing relationship to the Chætopoda. Simple trilobites with few segments, like the Agnostidæ, Eodiscidæ etc., were, therefore, properly placed by Beecher at the base of his classification, and there is now less chance than ever that they can be called degenerate animals.

From the phylogeny of certain groups, such as the Asaphidæ, it is learned that the geologically older members of the family have more strongly segmented anterior and posterior shields than the later ones. That there has been a "smoothing out" is demonstrated by a study of the ontogeny of the later forms. From such examples it has come to be thought that all smooth trilobites are specialized and occupy a terminal position in their genealogical line. This has caused some wonder that smooth agnostids like _Phalacroma bibullatum_ and _P. nudum_ should be found in strata so old as the Middle Cambrian, and was a source of great perplexity to me in the case of _Weymouthia_ (Ottawa Nat., vol. 27, 1913) (fig. 35). This is a smooth member of the Eodiscidæ, and, in fact, one of the simplest trilobites known, for while it has three thoracic segments, it shows almost no trace of dorsal furrows or segmentation on cephalon or pygidium, and, of course, no eyes. Following the general rule, I took this to be a smooth-out eodiscid, and was surprised that it should come from the Lower Cambrian, where it is associated with _Elliptocephala_ at Troy, New York, and with _Callavia_ at North Weymouth, Massachusetts, and where it has lately been found by Kiær associated with _Holmia_ and _Kjerulfia_ at Tømten, Norway. It now appears it is really in its proper zone, and instead of being the most specialized, is the simplest of the Eodiscidæ.

What appears to be a still simpler trilobite is the form described by Walcott as Naraoia.

=Naraoia compacta= Walcott.

(Text fig. 36.)

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 175, pl. 28, figs. 3, 4.--Cleland, Geology, Physical and Historical, New York, 1916, p. 412, fig. 382 F (somewhat restored).

This very imperfectly known form is referred by Walcott to the Notostraca on what appear to be wholly inadequate grounds, and while I do not insist on my interpretation, I can not refrain from calling attention to the fact that it _can_ be explained as the most primitive of all trilobites. It consists of two subequal shields, the anterior of which shows slight, and the posterior considerable evidence of segmentation. It has no eyes, no glabella, and no thorax, and is directly comparable to a very young _Phalacroma bibullatum_ (see Barrande 1852, pl. 49, figs. a, b). Walcott states that there is nothing to show how many segments there are in the cephalic shield, but that on one specimen fourteen were faintly indicated on the abdominal covering. The appendages are imperfectly unknown, as no specimen showing the ventral side has yet been described. The possible presence of antennas and three other appendages belonging to the cephalic shield is mentioned, and there are tips of fourteen legs projecting from beneath the side of one specimen. As figured, some of the appendages have the form of exopodites, others of endopodites, indicating that they were biramous.

_Naraoia_ is, so far as now known, possessed of no characteristics which would prevent its reference to the Trilobita, while the presence of a large abdominal as well as a cephalic shield would make it difficult to place in even so highly variable a group as the Branchiopoda. On the other hand, its only exceptional feature as a trilobite is the lack of thorax, and all study of the ontogeny of the group has led us to expect just that sort of a trilobite to be found some day in the most ancient fossiliferous rocks. _Naraoia_ can, I think, be best explained as a trilobite which grew to the adult state without losing its protaspian form. It was found in the Middle Cambrian of British Columbia.

Even if _Naraoia_ should eventually prove to possess characteristics which preclude the possibility of its being a primitive trilobite, it at least represents what I should expect a pre-Cambrian trilobite to look like. What the ancestry of the nektonic primitive trilobite may have been is not yet clear, but all the evidence from the morphology of cephalon, pygidium, and appendages indicates that it was a descendant of a swimming and not a crawling organism.

Since the above was written, the Museum of Comparative Zoology has purchased a specimen of this species obtained from the original locality. The shields are subequal, the posterior one slightly the larger, and the axial lobes are definitely outlined on both. The glabella is about one third the total width, nearly parallel-sided, somewhat pointed at the front. There are no traces of glabellar furrows. The axial lobe of the pygidium is also about one third the total width, extends nearly to the posterior margin, and has a rounded posterior end. The measurements are as follows: Length, 33 mm.; length of cephalon, 16 mm., width, 15 mm.; length of glabella, 11.5 mm., width, 5.5 mm.; length of pygidium, 17 mm., width, 15 mm.; length of axial lobe, 14 mm., width, 5.5 mm.

The species is decidedly _Agnostus_-like in both cephalon and pygidium, and were it not so large, might be taken for the young of such a trilobite. The pointed glabella is comparable to the axial lobes of the so-called pygidia of the young of _Condylopyge rex_ and _Peronopsis integer_ (Barrande, Syst. Sil., vol. 1, pl. 49).

The Ancestor of the Trilobites, and the Descent of the Arthropoda.

The "annelid" theory of the origin of the Crustacea and therefore of the trilobites, originating with Hatschek (1877) and so ably championed by Bernard (1892), has now been a fundamental working hypothesis for some years, and has had a profound influence in shaping thought about trilobites. This hypothesis has, however, its weak points, the principal one being its total inhibition of the workings of that great talisman of the palæontologist, the law of recapitulation. Its acceptance has forced the zoologist to look upon the nauplius as a specially adapted larva, and has caused more than one forced explanation of the protaspis of the trilobite. When so keen a student as Calman says that the nauplius must point in some way to the ancestor of the Crustacea (1909, p. 26), it is time to reëxamine some of the fundamentals. This has been done in the preceding pages and evidence adduced to show that the primitive features of a trilobite indicate a swimming animal, and that the adaptations are those which enabled it to assume a crawling mode of existence. It has also been pointed out that in Naraoia there is preserved down to Middle Cambrian times an animal like that to which ontogeny points as a possible ancestor of the trilobites. _Naraoia_ is not the simplest conceivable animal of its own type, however, for it has built up a pygidium of fourteen or fifteen somites. One would expect to find in Proterozoic sediments remains of similar animals with pygidia composed of only one or two somites, with five pairs of appendages on the cephalon, one or two pairs on the pygidium, a ventral mouth, and a short hypostoma. Anything simpler than this could not, in my opinion, be classed as a trilobite.

What the ancestor of this animal was is mere surmise. It probably had no test, and it may be noted in this connection that _Naraoia_ had a very thin shell, as shown by its state of preservation, and was in that respect intermediate between the trilobite and the theoretical ancestor. Every analysis of the cephalon of the trilobite shows that it is made up of several segments, certainly five, probably six, possibly seven. Every study of the trilobite, whether of adult, young, or protaspis, indicates the primitiveness of the lateral extensions or pleural lobes. The same studies indicate as clearly the location of the vital organs along the median lobe. These suggestions all point to a soft-bodied, depressed animal composed of few segments, probably with simple marginal eyes, a mouth beneath the anterior margin, tactile organs at one or both ends, with an oval shape, and a straight narrow gut running from anterior mouth to terminal anus. The broad flat shape gives great buoyancy and is frequently developed in the plankton. Inherited by the trilobites, it proved of great use to the swimmers among them.

The known animal which most nearly approaches the form which I should expect the remote ancestor of the trilobites to have had is _Amiskwia sagittiformis_ Walcott (Smithson. Misc. Coll., vol. 57, 1911, p. 112, pl. 22, figs. 3, 4). This "worm" from the Middle Cambrian is similar in outline to the recent _Spadella_, and is referred by Walcott to the Chætognatha. It has a pair of lateral expansions and a flattened caudal fin, a narrow median alimentary canal, and a pair of rather long simple tentacles. With the exception of a thin septum back of the head, no traces of segmentation are shown.

Some time in the late pre-Cambrian, the pre-trilobite, which probably swam by rhythmic undulations of the body, began to come into occasional contact with a substratum, and two things happened: symmetrically placed, i. e., paired, appendages began to develop on the contact surface, and a test on the dorsal side. The first use of the appendages may have been in pushing food forward to the mouth, and for the greater convenience in catching such material, a fold in front of the mouth may have elongated to form the prototype of the hypostoma. At this time the substratum may not have been the ocean bottom at all, but the animals, still free swimmers, may have alighted at feeding time on floating algæ from the surface of which they collected their food. While the dorsal test was originally jointed at every segment, the undulatory mode of swimming seems to have given way to the method of sculling by means of the posterior end only, or by the use of the appendages, and the anterior segments early became fused together.

The result of the hardening of the dorsal test was of course to reduce to that extent the area available for respiration, and this function was now transferred in part to the limbs, which bifurcated, one branch continuing the food-gathering process and the other becoming a gill. The next step may have been the "discovery" of the ocean bottom and the tapping of an hitherto unexploited supply of food. Upon this, there set in those adaptations to a crawling mode of existence which are so well shown in the trilobite. The crawling legs became lengthened and took on a hardened test, the hypostoma was greatly elongated, pushing the mouth backward, and new segments were added to produce a long worm-like form which could adapt itself to the inequalities of the bottom. That the test of the appendages became hardened later than that of the body is shown by the specimens of Neolenus, in which the dorsal shell as preserved in the shale is thick and solid, while the test of the appendages is a mere film.

The late Proterozoic or very earliest Cambrian was probably the time of the great splitting up into groups. The first development seems to have been among the trilobites themselves, the Hypoparia giving rise to two groups with compound eyes, first the Opisthoparia and later the Proparia. About this same time the Copepoda may have split off from the Hypoparia, continuing in the pelagic habitat. At first, most of the trilobites seem to have led a crawling existence, but about Middle Cambrian time they began to go back partially to the ancestral swimming habits, and retained some of the trunk segments to form a larger pygidium. The functional importance of the pygidium explains why it can not be used successfully in making major divisions in classification. Nearly related trilobites may be adapted to diverse methods of life.

EVOLUTION WITHIN THE CRUSTACEA.

The question naturally arises as to whether the higher Crustacea were derived from some one trilobite, or whether the different groups have been developed independently from different stocks. The opinion that all other crustaceans could have been derived from an _Apus_-like form has been rather generally held in recent years, but Carpenter (1903, p. 334) has shown that the leptostracan, _Nebalia_, is really a more primitive animal than _Apus_. He has pointed out that in Leptostraca the thorax bears eight pairs of simple limbs with lamelliform exopodites and segmented endopodites, while the abdomen of eight segments has six pairs of pleopods and a pair of furcal processes, so that only one segment is limbless. Contrasted with this are the crowded and complicated limbs of the anterior part of the trunk of _Apus_, and the appendage-less condition of the hinder portion. Further, a comparison between the appendages of the head of _Nebalia_ and those of _Apus_ shows that the former are the more primitive. The antennules of Nebalia are elongate, those of _Apus_ greatly reduced; the mandible of _Nebalia_ has a long endopodite, and Carpenter points out that from it either the malacostracan mandible with a reduced endopodite or the branchiopodan mandible with none could be derived, but that the former could not have arisen from the latter. The maxillæ of _Apus_ are also much the more specialized and reduced.

_Nebalia_ being in all else more primitive than _Apus_, it follows that the numerous abdominal segments of the latter may well have arisen by the multiplication of an originally moderate number, and the last trace of primitiveness disappears.

It is now possible to add to the results obtained from comparative morphology the testimony of palæontology, already outlined above, and since the two are in agreement, it must be admitted that the modern Branchiopoda are really highly specialized.

As has already been pointed out, _Hymenocaris_, the leptostracan of the Middle Cambrian, has very much the same sort of appendages as the Branchiopoda of the same age, both being of the trilobite type. Which is the more primitive, and was one derived from the other?

The Branchiopoda were much more abundant and much more highly diversified in Cambrian times than were the Leptostraca, and, therefore, are probably older. Some of the Cambrian branchiopods were without a carapace, and some were sessile-eyed. These were more trilobite-like than Hymenocaris. Many of the Cambrian branchiopods had developed a bivalved carapace, though not so large a one as that of the primitive Leptostraca. The present indications are, therefore, that the Branchiopoda are really older than the Leptostraca, and also that the latter were derived from them. It seems very generally agreed that the Malacostraca are descended from the Leptostraca, and the fossils of the Pennsylvanian supply a number of links in the chain of descent. Thus, _Pygocephalus cooperi_, with its brood pouches, is believed by Calman (1909, p. 181) to stand at the base of the Peracaridan series of orders, and _Uronectes_, _Palæocaris_, and the like are Palæozoic representatives of the Syncarida. Others of the Pennsylvanian species appear to tend in the direction of the Stomatopoda, whose true representatives have been found in the Jurassic. The Isopoda seem to be the only group of Malacostraca not readily connected up with the Leptostraca. Their depressed form, their sessile-eyes, and their antiquity all combine to indicate a separate origin for the group, and it has already been pointed out how readily they can be derived directly from the trilobite.

While the Copepoda seem to have been derived directly from the Hypoparia, the remainder of the Crustacea apparently branched off after the compound eyes became fully developed, unless, as seems entirely possible, compound eyes have been developed independently in various groups. Most Crustacea were derived from crawling trilobites (Lower Cambrian or pre-Cambrian Opisthoparia), for they lost the large pygidium, and also the major part of the pleural lobes. In all Crustacea, too, other than the Copepoda and Ostracoda, there is a tendency to lose the exopodites of the antennæ.

These modifications, which produced a considerable difference in the general appearance of the animal, are easily understood. As has been shown in previous pages, the trilobites themselves exhibit the degenerative effect on the anterior appendages of the backward movement of the mouth, and the transformation of a biramous appendage with an endobase into a uniramous antenna is a simple result of such a process. The feeding habits of the trilobites were peculiar and specialized, and it is natural that some members of the group should have broken away from them. In any progressive mode of browsing the hypostoma was a hindrance, so was soon gotten rid of, and the endobases not grouped around the mouth likewise became functionless. The chief factor in the development of the higher Crustacea seems to have been the pinching claw, by means of which food could be conveyed to the mouth. It had the same place in crustacean development that the opposable thumb is believed to have had in that of man.

An intermediate stage between the Trilobita and the higher Crustacea is at last exhibited to us by the wonderful, but unfortunately rather specialized _Marrella_, already described. It retains the hypostoma and the undifferentiated biramous appendages of the trilobite, but has uniramous antennæ, there are no endobases on the coxopodites of the thoracic appendages, the pygidium is reduced to a single segment, and the lateral lobes of the thorax are also much reduced. _Marrella_ is far from being the simplest of its group, but is the only example which survived even down to Middle Cambrian times of what was probably once an important series of species transitional between the trilobites and the higher Crustacea.

In this theory of the origin of the Crustacea from the Trilobita, the nauplius becomes explicable and points very definitely to the ancestor. According to Calman (1909, p. 23):

The typical nauplius has an oval unsegmented body and three pairs of limbs, corresponding to the antennules, antennas, and mandibles of the adult. The antennules are uniramous, the others biramous, and all three pairs are used in swimming. The antennæ may have a spiniform or hooked masticatory process at the base, and share with the mandibles which have a similar process, the function of seizing and masticating the food. The mouth is overhung by a large labrum or upper lip and the integument of the dorsal surface of the body forms a more or less definite dorsal shield. The paired eyes are as yet wanting, but the median eye is large and conspicuous.

The large labrum or hypostoma, the biramous character of the appendages, especially of the antennæ, the functional gnathobases on the second and third appendages, and the oval unsegmented shield are all characteristics of the trilobites, and it is interesting to note that all nauplii have the free-swimming habit.

The effect of inheritance and modification through millions of generations is also shown in the nauplius, but rather less than would be expected. The most important modification is the temporary suppression of the posterior pairs of appendages of the head, so that they are generally developed later than the thoracic limbs. The median or nauplius eye has not yet been found in trilobites, and if it is, as it appears to be, a specialized eye, it has probably arisen since the later Crustacea passed the trilobite stage in their phylogeny.

The oldest Crustacea, other than trilobites, so far known are the Branchiopoda and Phyllocarida described by Walcott and discussed above. It is important to note that while the former have already achieved such modified characteristics that they have been referred to modern orders, they retain the trilobite-like limbs and some of them still have well developed pleural lobes.

Calman (1909, p. 101) says of the Copepoda:

On the hypothesis that the nauplius represents the ancestral type of the Crustacea, the Eucopepoda would be regarded as the most primitive existing members of the class, retaining as they do, naupliar characters in the form of the first three pairs of appendages and in the absence of paired eyes and of a shell-fold. As already indicated, however, it is much more probable that they are to be regarded as a specialized and in some respects degenerate group which, while retaining, in some cases, a very primitive structure of the cephalic appendages, has diverged from the ancestral stock in the reduction of the number of somites, the loss of the paired eyes and the shell-fold, and the simplified form of the trunk-limbs.

If the Eucopepoda be viewed in the light of the theory of descent here suggested, it is at once seen that while they are modified and specialized, they more nearly approximate the hypothetical ancestor than any other living Crustacea. Compound eyes are absent, and it can not be proved that they were ever present, although Grobben is said to have observed rudiments of them in the development of _Calanus_. The "simplified limbs" are the simple limbs of the trilobite, somewhat modified. The absence of the shell-fold and carapace is certainly a primitive characteristic. Add to this the direct development of the small number of segments, and the infolded pleural lobes, and it must be admitted that the group presents more trilobite-like characteristics than any other. It seems very likely that the primitive features were retained because of the pelagic habitat of a large part of the group.

Ruedemann (Proc. Nat. Acad. Sci., vol. 4, 1918, p. 382, pl.) has recently outlined a possible method of derivation of the acorn barnacles from the phyllocarids. Starting from a recent _Balanus_ with rostrum and carina separated by two pairs of lateralia, he traces back through _Calophragmus_ with three pairs of lateralia to _Protobalanus_ of the Devonian with five pairs. Still older is the newly discovered _Eobalanus_ of the upper Ordovician, which also has five pairs of lateralia but the middle pair is reversed, so that when the lateralia of each side are fitted together, they form a pair of shields like those of _Rhinocaris_, separated by the rostrum and carina, which are supposed to be homologous with the rostrum and dorsal plate of the Phyllocarida. Ruedemann suggests that the ancestral phyllocarid attached itself by the head, dorsal side downward, and the lateralia were developed from the two valves of the carapace during its upward migration, to protect the ventral side exposed in the new position.

This theory is very ingenious, but has not been fully published at the time of writing, and it seems very doubtful if it can be sustained.

_Summary._

The salient points in the preceding discussion should be disentangled from their setting and put forward in a brief summary.

It is argued that the ancestral arthropod was a short and wide pelagic animal of few segments, which so far changed its habits as to settle upon a substratum. As a result of change in feeding habits, appendages were developed, and, due perhaps to physiological change induced by changed food, a shell was secreted on the dorsal surface, covering the whole body. Such a shell need not have been segmented, and, in fact, the stiffer the shell, the more reason for development of the appendages. Activity as a swimming and crawling animal tended to break up the dorsal test into segments corresponding to those of the soft parts, and, by adaptation, a floating animal became a crawling one, with consequent change from a form like that of _Naraoia_ to one like _Pædeumias_. (See figs. 36-40.) A continuation of this line of development by breaking up and loss of the dorsal test led through forms similar to _Marrella_ to the Branchiopoda of the Cambrian, in which not only is there great reduction in the test, but also loss of appendages. The origin of the carapace is still obscure, but Bernard (1892, p. 214, fig. 48) has already pointed out that some trilobites, Acidaspidæ particularly, have backward projecting spines on the posterior margin of the cephalon, which suggest the possibility of the production of such a shield, and in _Marrella_ such spines are so extravagantly developed as almost to confirm the probability of such origin. In this line of development two pairs of tactile antennæ were produced, while the anomomeristic character of the trilobite was retained. From similar opisthoparian ancestors there were, however, derived primitive Malacostraca retaining biramous antennæ, but with a carapace and reduced pleural lobes and pygidium. From this offshoot were probably derived the Ostracoda, the Cirripedia, and the various orders of the Malacostraca, with the possible exception of the Isopoda. I have suggested independent origins of the Copepoda and Isopoda, but realize the weighty arguments which can be adduced against such an interpretation.

It is customary to speak of the Crustacea and Trilobita as having had a common ancestry, rather than the former being in direct line of descent from the latter, but when it can be shown that the higher Crustacea are all derivable from the Trilobita, and that they possess no characteristics which need have been inherited from any other source than that group, it seems needless to postulate the evolution of the same organs along two lines of development.

I can not go into the question of which are more primitive, sessile or stalked eyes, but considering the various types found among the trilobites, one can but feel that the stalked eyes are not the most simple. While no trilobite had movable stalked eyes, it is possible to homologize free cheeks with such structures. They always bear the visual surface, and, in certain trilobites (_Cyclopyge_), the entire cheek is broken up into lenses. Since a free cheek is a separate entity, it is conceivable that it might lie modified into a movable organ.

EVOLUTION OF THE MEROSTOMATA.

It has been pointed out above that the Limulava (_Sidneyia_, _Amiella_, _Emeraldella_) have certain characteristics in common with the trilobites on the one hand and the Eurypterida on the other. These relationships have been emphasized by Walcott, who derives the Eurypterida through the Limulava and the Aglaspina from the Trilobita. The Limulava may be derived from the Trilobita, but indicate a line somewhat different from that of the remainder of the Crustacea. In this line the second cephalic appendages do not become antennæ and the axial lobe seems to broaden out, so that the pleural lobes become an integral part of the body. As in the modern Crustacea, the pygidium is reduced to the anal plate, and this grows out into a spine-like telson.

From the Limulava to the Eurypterida is a long leap, and before it can be made without danger, many intermediate steps must be placed in position. The direct ancestor of the Eurypterida is certainly not to be seen in the highly specialized _Sidneyia_, and probably not in _Emeraldella_, but it might be sought in a related form with a few more segments. The few species now known do suggest the beginning of a grouping of appendages about the mouth, a suppression of appendages on the abdomen, and a development of gills on the thorax only. Further than that the route is uncertain.

Clarke and Ruedemann, whose recent extensive studies give their opinion much weight, seem fully convinced that the Merostomata could not have been derived from the Trilobita, but are rather inclined to agree with Bernard that the arachnids and the crustaceans were derived independently from similar chætopod annelids (1912, p. 148).

The greater part of their work was, however, finished before 1910, and although they refer to Walcott's description of the Limulava (1911), they did not have the advantage of studying the wonderful series of Crustacea described by him in 1912. While the evidence is far from clear, it would appear that the discovery of animals with the form of Limiting and the eurypterids and the appendages of trilobites means something more than descent from similar ancestors. Biramous limbs of the type found in the trilobites would probably not be evolved independently on two lines, even if the ancestral stocks were of the same blood.

The Aglaspidæ, as represented by _Molaria_ and _Habelia_ in the Middle Cambrian, are quite obvious closely related to the trilobites easily derived from them, and retain numerous of their characteristics. That they are not trilobites is, however, shown by the presence of two pairs of antennæ, the absence of facial sutures, and the possession of a spine-like telson.

The Aglaspidæ have always been placed in the Merostomata, and nearer the Limulidæ than the Eurypterida. The discovery of appendages does not at all tend to strengthen that view, but indicates rather that they are true Crustacea which have not given rise to any group now known. The exterior form is, however, _Limulus_-like, and since it is known from ontogeny that the ancestor of that genus was an animal with free body segments, there is still a temptation to try to see in the Aglaspidæ the progenitors of the limulids.

The oldest known _Limulus_-like animal other than the Aglaspidæ is _Neolimulus falcatus_ Woodward (Geol. Mag., dec. 1, vol. 5, 1868, p. 1, pl. 1, fig. 1). The structure of the head of this animal is typically limuloid, with simple and compound eyes and even the ophthalmic ridges. Yet, curiously enough, it shows what in a trilobite would be considered the posterior half of the facial suture, running from the eye to the genal angle. The body is composed of eight free segments with the posterior end missing. _Belinurus_, from the Mississippian and Pennsylvanian, has a sort of pygidium, the posterior three segments being fused together, and _Prestwichia_ of the Pennsylvanian has all the segments of the abdomen fused together. So far as form goes, a very good series of stages can be selected, from the Aglaspidæ of the Cambrian through _Neolimulus_ to the Belinuridæ of the late Palæozoic and the Limulidæ of the Mesozoic to recent. Without much more knowledge of the appendages than is now available, it would be quite impossible to defend such a line. It is, however, suggestive.

EVOLUTION OF THE "TRACHEATA."

The trilobites were such abundant and highly variable animals, adapting themselves to various methods of life in the sea, that it appears highly probably that some of them may have become adapted to life on the land. The ancestors of the Chilopoda, Diplopoda, and Insecta appear to have been air-breathing animals as early as the Cambrian, or at latest, the Ordovician. Since absolutely nothing is yet known of the land or even of the fresh-water life of those periods, nothing can now be proved.

In discussing the relationship of the trilobites to the various tracheate animals, I have pointed out such palæontologic evidence as I have been able to gather. Studies in the field of comparative morphology do not fall within my province. I only hope to have made the structure of the trilobite a little more accessible to the student of phylogenies.

SUMMARY ON LINES OF DESCENT.

In order to put into graphic and concise form the suggestions made above, it is necessary to define and give names to some of the groups outlined. The hypothetical ancestor need not be included in the classification and for reasons of convenience may be referred to merely as the Protostracean.

The group of free-swimming trilobites without thoracic segments was probably a large one, and within it there were doubtless considerable variations and numerous adaptations. While the only known animal which could possibly be referred to this group, _Naraoia_, is blind, it is entirely possible that other species had eyes, and that the cephala and pygidia were variously modified. For this reason and because of the lack of all thoracic segments, it seems better to erect a new order rather than merely a family for the group, and _Nektaspia_ (swimming shields) may be suggested. The only known family is Naraoidæ Walcott, which must be redefined.

_Marrella_ and _Habelia_ are types of Crustacea which can neither be placed in the Trilobita nor in any of the established subclasses of the Eucrustacea. They represent a transitional group, the members of which are, so far as known, adapted to the crawling mode of life, though it may prove that there are also swimmers which can be classified with them. To this subclass the name _Haplopoda_ may be applied, the feet being simple.

The two known families, Marrellidæ Walcott and Aglaspidæ Clarke, belong to different orders, the second having already the name Aglaspina Walcott. The name _Marrellina_ may therefore be used for the other.

For _Sidneyia_, Walcott proposed the new subordinal name Limulava, placing it under the Eurypterida. While _Sidneyia_, _Emeraldella_, and _Amiella_ may belong to the group that gave rise to the Eurypterida, they are themselves Crustacea, and a place must be found for them in that group. The possession of only one pair of antennæ prevents their reception by the Haplopoda, and allies them to the Trilobita, but the modifications of the trunk and its appendages keep them out of that subclass, and a new one has to be erected for them. This may be known as the _Xenopoda_, in allusion to the strange appendages of _Sidneyia_.

_Synopsis._

Class Crustacea.

Subclass Trilobita Walch.

Crustacea with one pair of uniramous antennæ, and possessing facial sutures.

Order Nektaspia nov.

Trilobita without thoracic segments. Cephala and pygidia simple.

Family Naraoidæ Walcott.

Cephalon and pygidium large, both shields nearly smooth. Eyes absent. A single species: _Naraoia compacta_ Walcott, Middle Cambrian, British Columbia.

Subclass Haplopoda nov.

Crustacea with trilobate form, two pairs of uniramous antennæ, no facial sutures, sessile compound eyes present or absent, pygidium and pleural lobes generally reduced, large labrum present, appendages of the trunk biramous.

Order Marrellina nov.

Form trilobite-like, pleural lobes reduced, endobases absent from coxopodites of body, pygidium a small plate.

Family Marrellidæ Walcott.

Cephalon with long genal and nuchal spines. Eyes marginal. A single species: _Marrella splendens_ Walcott, Middle Cambrian, British Columbia.

Order Aglaspina Walcott.

Body trilobite-like, with few thoracic segments, and a spine-like telson. Appendages biramous.

Family Aglaspidæ Clarke.

Cephalon trilobate, with or without compound eyes, seven or eight segments in the thorax.

Genus _Aglaspis_ Hall.

Compound eyes present, seven segments in thorax. Upper Cambrian, Wisconsin.

Genus _Molaria_ Walcott.

Compound eyes absent, eight segments in thorax. Middle Cambrian, British Columbia.

Genus _Habelia_ Walcott.

Compound eyes absent. Not yet fully described. Middle Cambrian, British Columbia.

Subclass Xenopoda nov.

Crustacea with more or less eurypterid-like form, one pair of uniramous antennæ, biramous appendages on anterior part of trunk, modified endopodites on cephalon.

Order Limulava Walcott.

Cephalon with lateral or marginal eyes and large epistoma. Body with eleven free segments and a telson. Cephalic appendages grouped about the mouth.

Family Sidneyidæ Walcott.

Trunk probably with exopodites only, and without appendages on the last two segments. Telson with a pair of lateral swimmerets.

Genus _Sidneyia_ Walcott.

Third cephalic appendage a large compound claw. Gnathobases forming strong jaws. Middle Cambrian, British Columbia.

Genus _Amiella_ Walcott.

Middle Cambrian, British Columbia.

Family Emeraldellidæ nov.

Trunk with biramous appendages in anterior part, and appendages on all segments except possibly the spine-like telson.

Genus _Emeraldella_ Walcott.

Cephalic appendages simple spiniferous endopodites. Eyes unknown. Middle Cambrian, British Columbia.

Final Summary.

It is generally believed that the Arthropoda constitute a natural, monophyletic group. The data assembled in the preceding pages indicate that the other Arthropoda were derived directly or indirectly from the Trilobita because:

(1) the trilobites are the oldest known arthropods;

(2) the trilobites of all formations show great variation in the number of trunk segments, but with a tendency for the number to become fixed in each genus;

(3) the trilobites have a constant number of segments in the head;

(4) the position of the mouth is variable, so that either the Crustacea or the Arachnida could be derived from the trilobites;

(5) the trilobite type of appendage is found, in vestigial form at least, throughout the Arthropoda;

(6) the appendages of all other Arthropoda are of forms which could have been derived from those of trilobites;

(7) the appendages of trilobites are the simplest known among the Arthropoda;

(8) the trilobites show practically all known kinds of sessile arthropodan eyes, simple, compound, and aggregate;

(9) the apparent specializations of trilobites, large pleural lobes and pygidia, are primitive, and both suffer reduction within the group.

The ancestor of the trilobite is believed to have been a soft-bodied, free-swimming, flat, blind or nearly blind animal of few segments, because:

(a) the form of both adult and embryo is of a type more adapted for floating than crawling;

(b) the large pygidium is shown by ontogeny to be primitive, and the elongate worm-like form secondary;

(c) the history of the trilobites shows a considerable increase in the average number of segments in successive periods from the Cambrian to the Permian;

(d) the simplest trilobites are nearly or quite blind.