The History of Creation, Vol. 2 (of 2) Or the Development of the Earth and its Inhabitants by the Action of Natural Causes

CHAPTER XIX.

Chapter 168,946 wordsPublic domain

PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.

II. MOLLUSCA, STAR-FISHES, AND ARTICULATED ANIMALS.

Tribe of Molluscs.—Four Classes of Molluscs: Lamp-shells (Spirobranchia); Mussels (Lamellibranchia); Snails (Cochlides); Cuttle-fish (Cephalopoda).—Tribe of Star-fishes, or Echinoderma.—Their Derivation from Ringed Worms (Mailed Worms, or Phracthelminthes).—The Alternation of Generation in the Echinoderma.—Four Classes of Star-fish: Sea-stars (Asteridea); Sea-lilies (Crinoidea); Sea-urchins (Echinidea); Sea-cucumbers (Holothuridea).—Tribe of Articulated Animals, or Arthropoda.—Four Classes of Articulated Animals: Branchiata, or Crustacea, breathing through gills; Jointed Crabs; Mailed Crabs; Articulata Tracheata, breathing through Air Tubes.—Spiders (Long Spiders, Round Spiders).—Myriopods.—Insects.—Chewing and Sucking Insects.—Pedigree and History of the Eight Orders of Insects.

The great natural main groups of the animal kingdom, which we have distinguished as TRIBES, or PHYLA (“types” according to Bär and Cuvier), are not all of equal systematic importance for our phylogeny or history of the pedigree of the living world. They can neither be classed in a single series of stages, one above another, nor be considered as entirely independent stems, nor as equal branches of a single family-tree. It seems rather (as we saw in the last chapter) that the tribe of Protozoa, the so-called primæval animals, is the common radical group of the whole animal kingdom. Out of the Gastræada—which we class among the Protozoa—the Zoophytes and the Worms have developed, as two diverging branches. We must now in turn look upon the varied and much-branching tribe of Worms as the common primary group, out of which (from perfectly distinct branches) arose the remaining tribes, the four higher phyla of the animal kingdom. (Compare the Pedigree, p. 133.)

Let us now take a genealogical look at these four higher tribes of animals, and try whether we cannot make out the most important outlines of their pedigree. Even should this attempt prove defective and imperfect, we shall at all events have made a beginning, and paved the road for subsequent and more satisfactory attempts.

It does not matter in what succession we take up the examination of the four higher tribes. For these four phyla have no close relationship whatever among one another, but have grown out from entirely distinct branches of the group of Worms (p. 133). We may consider the tribe of Molluscs as the most imperfect and the lowest in point of morphological development. We nowhere meet among them with the characteristic articulation or segmented formation of the body, which distinguishes even the Ring-worms, and which in the other three higher tribes—the Echinoderma, Articulata, and Vertebrata—is most essentially connected with the high development of their forms, their differentiation, and perfection. The body in all Molluscs—in mussels, snails, etc.—is a simple non-jointed sack, in the cavity of which lie the intestines. The nervous system consists not of a cord but of several distinct (generally three) pairs of knots loosely connected with one another. For these and many other anatomical reasons, I consider the tribe of Molluscs (in spite of the high physiological development of its most perfect forms) to be morphologically the lowest among the four higher tribes of animals.

Whilst, for reasons already given, we exclude the Moss-polyps, and Tunicates—which have hitherto been generally classed with the tribe of Molluscs—we retain as genuine Molluscs the following four classes: Lamp-shells, Mussels, Snails, and Cuttles. The two lower classes of Molluscs, the Lamp-shells and Mussels, possess neither head nor teeth, and they can therefore be comprised under one main class, or branch, as headless animals (Acephala), or toothless animals (Anodontoda). This branch is also frequently called that of the clam-shells (Conchifera, or Bivalvia), because all its members possess a two-valved calcareous shell. In contrast to these the two higher classes of Molluscs, the snails and cuttles, may be represented as a second branch with the name of Head-bearers (Cephalophora), or Tooth-bearers (Odontophora), because both head and teeth are developed in them.

The soft, sack-shaped body in most Molluscs is protected by a calcareous shell or house, which in the Acephala (lamp-shells and mussels) consists of two valves, but in the Cephalophora (snails and cuttles) is generally a spiral tube (the so-called snail’s house). Although these hard skeletons are found in large quantities in a petrified state in all the neptunic strata, yet they tell us but little of the historical development of the tribe, which must have taken place for the most part in the primordial period. Even in the Silurian strata we find fossil remains of all the four classes of Molluscs, one beside the other, and this, conjointly with much other evidence, distinctly proves that the tribe of Molluscs had then obtained a strong development, when the higher tribes, especially the Articulates and Vertebrates, had scarcely got beyond the beginning of their historical development. In subsequent periods, especially in the primary and secondary periods, these higher tribes increased in importance more and more at the expense of Molluscs and Worms, which were no match for them in the struggle for life, and accordingly decreased in number. The still living Molluscs and Worms must be considered as only a proportionately small remnant of the vast molluscan fauna, which greatly predominated in the primordial and primary periods over the other tribes. (Compare Plate VI. and explanation in the Appendix.)

No tribe of animals shows more distinctly than do the Molluscs, how very different the value of fossils is in geology and in phylogeny. In geology the different species of the fossil shells of Molluscs are of the greatest importance because they serve as excellent marks whereby to characterize the different groups of strata, and to fix their relative ages. As far as relates to the genealogy of Molluscs, however, they are of very little value, because, on the one hand, the shells are parts of quite subordinate morphological importance, and because the actual development of the tribe belongs to the earlier primordial period, from which no distinct fossils have been preserved. If therefore we wish to construct the pedigree of Molluscs, we are mainly dependent upon the records of ontogeny and comparative anatomy from which we obtain something like the following result. (Gen. Morph. ii. Plate VI. pp. 102-116.)

The lowest stage of the four classes of genuine Molluscs known to us, is occupied by the Lamp-shells or Spiral-gills (Spirobranchia), frequently but inappropriately called Arm-footers (Brachiopoda), which have become attached to the bottom of the sea. There now exist but few forms of this class; for instance, some species of Lingula, Terebratula, and others akin to them, which are but feeble remnants of the great variety of forms which represented the Lamp-shells in earlier periods of the earth’s history. In the Silurian period they constituted the principal portion of the whole Mollusc tribe. From the agreement which, in many respects, their early stage of development presents with the Moss animals, it has been concluded that they have developed out of Worms, which were nearly related to this class. Of the two sub-classes of Lamp-shells, the Hinge-less (Ecardines) must be looked upon as the lower and more imperfect, the Hinged (Testicardines) as the higher and more fully developed group.

The anatomical difference between the Lamp-shells and the three other classes of Molluscs is so considerable that the latter may be distinguished from the former by the name of Otocardia. All the Otocardia have a heart with chamber (ventricle) and ante-chamber (auricle), whereas Lamp-shells do not possess the ante-chamber. Moreover, the central nervous system is developed only in the former (and not in the latter) in the shape of a complete pharyngeal ring. Hence the four classes of Molluscs may be grouped in the following manner:—

{ 1. Lamp-shells } I. Haplocardia I. Molluscs { (Spirobranchia). } (with simple heart). without head. { _Acephala._ { 2. Mussels } { (Lamellibranchia). } II. Otocardia } (with chamber II. Molluscs { 3. Snails } and ante-chamber with head. { (Cochlides). } to the heart). _Cephalophora._ { 4. Cuttles } { (Cephalopoda). }

The result of these structural dispositions for the history of the pedigree of Molluscs, which is confirmed by palæontology, is that Lamp-shells stand much nearer to the primæval root of the whole tribe of Molluscs than do the Otocardia. Probably Mussels and Snails developed as two diverging branches out of Molluscs, which were nearly akin to the Lamp-shells.

Mussels, or Plate-gills (Lamellibranchia), possess a bivalved shell like the Lamp-shells. In the latter, one of the two valves covers the back, the other the belly of the animal; whereas in Mussels the two valves lie symmetrically on the right and left side of the body. Most Mussels live in the sea, only a few in fresh water. The class is divided into two sub-classes, Asiphonia and Siphonida, of which the latter were developed at a later period out of the former. Among the Asiphonia are Oysters, mother-of-pearl Shells, and fresh water Mussels; among the Siphonida, which are characterized by a respiratory tube, are the Venus-shells, Razor-shells, and Burrowing Clams. The higher Molluscs seem to have developed at a later period out of those without head and teeth; they are distinguished from the latter by the distinct formation of the head, and more especially by a peculiar kind of tooth apparatus. Their tongue presents a curious plate, armed with a great number of teeth. In our common Vineyard Snail (Helix pomatia) the number of teeth amount to 21,000, and in the large Garden Slug (Limax maximus) to 26,800.

SYSTEMATIC SURVEY

_Of the 4 Classes, 8 Sub-classes, and 21 Orders of Molluscs._

------------------+-----------------------+------------------------+------------------- _Classes of_ | _Sub-classes of_ | _Orders of_ | _Systematic Name_ _Molluscs._ | _Molluscs._ | _Molluscs._ | _of the Orders._ ------------------+-----------------------+------------------------+------------------- I. _Molluscs without head or teeth_: ACEPHALA _or_ ANODONTODA. --------------------------------------------------------------------------------------- I. { I. Ecardines { 1. Stalked 1. Lingulida =Lamp-shells= { _Hinge-less_ { 2. Flattened 2. Craniada { +Spirobranchia+ { or { +Brachiopoda+ { II. Testicardines { 3. Fleshy armed 3. Sarcobrachia { _Hinge-less_ { 4. Calcareous-armed 4. Sclerobrachia

II. { III. Asiphonia { 5. One-muscled 5. Monomya =Mussels= { _Mussels without_ { 6. Uneven-muscled 6. Heteromya or { _respiratory tubes_ { 7. Even-muscled 7. Isomya =Plate-gills= { { +Lamellibranchia+ { IV. Siphonida { 8. Round-mantled 8. Integripallia or { _Mussels with_ { 9. Ray-mantled 9. Sinupalliata +Phyllobranchia+ { _respiratory tubes_ { 10. Tube-mussels 10. Inclusa

--------------------------------------------------------------------------------------- II. _Molluscs with head and teeth_: CEPHALOPHORA _or_ ODONTOPHORA. ---------------------------------------------------------------------------------------

{ V. Stump-headed { 11. Tube-snails 11. Scaphopoda III. { _Perocephala_ { 12. Butterfly-snails 12. Pteropoda =Snails= { { { 13. With hind gills 13. Opisthobranchia +Cochlides+ { { 14. With fore gills 14. Prosobranchia or { VI. Large-headed { 15. Swimming-snails 15. Heteropoda +Gasteropoda+ { _Delocephala_ { 16. Beetle-snails 16. Chitonoida { { 17. Snails with lungs 17. Pulmonata

VII. Chamber-Poulps { 18. Pearl boats 18. Nautilida IV. with four gills { 19. Ammon’s horns 19. Ammonitida =Cuttles= } _Tetrabranchia_ { or } =Poulps= } VIII. Ink-Poulps with { 20. Ten-armed 20. Decabrachiones } two gills { +Cephalopoda+ } _Dibranchia_ { 21. Eight-armed 21. Octobrachiones

----------------------------------------------------------------

We distinguish two sub-classes among the Snails (Cochlides, or Gasteropoda), namely, the Stump-headed and the Large-headed Snails. The Stump-headed Snails (Perocephala) are very closely allied to Mussels (through the Tooth-shells), and also to the Cuttle-fish (through the Butterfly-snails). The more highly developed Snails, with large heads (Delocephala), can be divided into Snails with gills (Branchiata) and Snails with lungs (Pulmonata). Among the latter are the Land-snails, the only Molluscs which have left the water and become habituated to a life on land. The great majority of Snails live in the sea, only a few live in fresh water. Some River-snails in the tropics (the Ampullaria) are amphibious, living sometimes on land, sometimes in water, and at one time they breathe through gills, at another through lungs. They have both kinds of respiratory organs, like the Mud-fish and Gilled Newts among the Vertebrata.

The fourth and last class, and at the same time the most highly developed class of Molluscs, is that of the Cuttles, or Poulps, also called Cephalopoda (foot attached to the head). They all live in the sea, and are distinguished from Snails by eight, ten, or more long arms, which surround the mouth in a circle. The Cuttles existing in our recent oceans—the Sepia, Calamary, Argonaut, and Pearly Nautilus—are, like the few Spiral-gill Lamp-shells of the present time, but a poor remnant of the host which represents this class in the oceans of the primordial, primary, and secondary periods. The numerous fossil “Ammon’s horns” (Ammonites), “pearl boats” (Nautilus), and “thunderbolts” (Belemnites) are evidences of the long since extinct splendour of the tribe. The Poulps, or Cuttles, have probably developed out of a low branch of the snail class, out of the Butterfly-snails (Pteropoda) or kindred forms.

The different sub-classes and orders, distinguished in the four classes of Molluscs, whose systematic succession is given on the Table (p. 160), furnish various proofs of the validity of the law of progress by their historical development and by the systematic development corresponding to it. As however these subordinate groups of Molluscs are in themselves of no further special interest, I must refer to the sketch of their pedigree on p. 161, and to the detailed pedigree of Molluscs which I have given in my General Morphology, and I shall now at once turn to the consideration of the tribe of Star-fishes.

The Star-fishes (Echinoderma, or Estrellæ) among which are the four classes of Sea-stars, Sea-lilies, Sea-urchins, and Sea-cucumbers are one of the most interesting divisions of the animal kingdom, and yet we know less about them than about any. They all live in the sea. Every one who has been at the sea shore must have seen at least two of their forms, the Sea-stars and the Sea-urchins. The tribe of Star-fishes must be considered as a completely independent tribe of the animal kingdom on account of its very peculiar organization, and must be carefully distinguished from the Animal-plants—Zoophytes, or Cœlenterata, with which it is still frequently but erroneously classed under the name Radiata (as for example, by Agassiz, who even to this day defends this error of Cuvier’s, together with many others).

All Echinoderma are characterized, and at the same time distinguished from all other animals, by a very remarkable apparatus for locomotion, which consists of a complicated system of canals or tubes, filled with sea water from without. The sea water in these aqueducts is moved partly by the strokes of the cilia, or vibratile hairs lining their walls, and partly by the contractions of the muscular walls of the tubes themselves, which resemble india-rubber bags. The water is pressed from the tubes into a number of little hollow feet, which thereby become widely distended, and are then employed for walking and suction. The Sea-stars are moreover characterized by a peculiar calcareous formation in the skin, which in most cases forms a firm, well-closed coat of mail, composed of a number of plates. In almost all Echinoderma the body consists of five radii (counterparts, or antimera) standing round the main axis of the body, where they meet. It is only in some species of Sea-stars that the number of these radii amount to more than five—to 6-9, 10-12, or even to 20-40; and in this case the number of radii is generally not constant, but varies in different individuals of one species.

The historical development and the pedigree of the Echinoderma are completely revealed to us by their numerous and, in most cases, excellently preserved fossil remains, by their very remarkable individual developmental history, and by their interesting comparative anatomy; this is the case with no other tribe of animals, even the Vertebrata themselves are not to be excepted. By a critical use of those three archives, and by a careful comparison of the results derived from their study, we obtain the following genealogy of the Star-fishes, which I have already published in my General Morphology (vol. ii. Plate IV. pp. 62-77.)

The most ancient and original group of the Star-fishes, the primary form of the whole phylum, consists of the class of the true Sea-stars (Asterida). This is established by numerous and important arguments in anatomy and the history of development, but above all by the irregular and varying number of the radii, or antimera, which in all other Echinoderma is limited, without exception, to five. Every Star-fish consists of a central, small, body-disc, all round the circumference of which are attached five or several long articulated arms. _Each arm of the Star-fish essentially corresponds in its organisation with an articulated worm_ of the class of Ring-worms, or Annelida (p. 149). I therefore consider the Star-fish as a genuine _stock or cormus of five or more articulated worms_, which have arisen by the star-wise growth of a number of buds out of a central mother-worm. The connected members, thus grouped like the rays of a star, have inherited from the mother-worm the common opening of the mouth, and the common digestive cavity (stomach) lying in the central body-disc. The end by which they have grown together, and which fuses in the common central disc, probably corresponds to the posterior end of the original independent worms.

In exactly the same way several individuals of certain kinds of worms are united so as to form a star-like cormus. This is the case in the _Botryllidæ_, compound Ascidians, belonging to the class of the Tunicata. Here also the posterior ends of the individual worms have grown together, and have formed a common outlet for discharges, a central cloaca; whereas at the anterior end each worm still possesses its own mouth. In Star-fishes the original mouths have probably become closed in the course of the historical development of the cormus, or colony, whereas the cloaca has developed into a common mouth for the whole cormus.

SYSTEMATIC SURVEY

_Of the 4 Classes, 9 Sub-classes, and 20 Orders of Star-fishes._

(Compare Gen. Morph. II. Plate IV. pp. 62-67.)

----------------+-------------------------+---------------------------+------------------ _Classes of the | _Sub-classes of the | _Orders of the | _Systematic Name Star-fishes._ | Star-fishes._ | Star-fishes._ | of the Orders._ ----------------+-------------------------+---------------------------+------------------ | { I. { 1. Primary Stars | 1. Tecastra { Sea Stars with radiated { 2. Articulated Stars | 2. Colastra I. { stomach { 3. Brisinga Stars | 3. Brisingastra =Sea Stars= { _Actinogastra_ | +Asterida+ { { II. { 4. Serpent Stars | 4. Ophiastra { Sea Stars with disc- { 5. Tree Stars | 5. Phytastra { shaped stomach { 6. Lily Stars | 6. Crinastra { _Discogastra_ { | | { III. { 7. Plated Lilies with | 7. Phatnocrinida { Lilies with arms { arms | { _Brachiata_ { 8. Articulated Lilies | 8. Colocrinida { { with arms | | II. { IV. { 9. Regularly budding | 9. Pentremitida =Sea Lilies= { Lilies with buds { Lilies | +Crinoida+ { _Blastoidea_ { 10. Lilies budding on | 10. Eleutherocrina { { two sides | | { V. { 11. Bladder Lilies | 11. Agelacrinida { Bladder Lilies { without stalks | { _Cystidea_ { 12. Bladder Lilies | 12. Sphæronitida { with stalks | | { VI. { 13. Palechinida with | 13. Melonitida { Older Sea Urchins { more than 10 | { (with more than { rows of ambulacral | { 20 rows of plates) { plates | III. { _Palechinida_ { 14. Palechinida with | 14. Eocidaria =Sea Urchins= { { 10 rows of | +Echinida+ { VII. { ambulacral plates | { More recent Sea { 15. Autechinida with | 15. Desmosticha { Urchins (with 20 { band-like ambulacra | { rows of plates) { 16. Autechnidia with | 16. Petalosticha { _Autechinida_ { leaf-like ambulacra | | { VIII. { 17. Eupodia with scuti- | 17. Aspidochirota { Sea Cucumbers { form tentacles | { with aquatic feet { 18. Eupodia with | 18. Dendrochirota IV. { _Eupodia_ { branching tentacles | =Sea Cucumbers= { { | { IX. { | +Holothuriæ+ { Sea Cucumbers { 19. Apodia with water- | 19. Liodermatida { without aquatic { lungs | { feet { 20. Apodia without | 20. Synaptida { _Apodia_ { water-lungs |

----------------------------------------------------------------

Hence the Star-fishes would be compound stocks of worms which, by the radial formation of buds, have developed out of true articulated worms, or Annelids. This hypothesis is most strongly supported by the comparative anatomy, and by the ontogeny of some Star-fishes (Colastra), and of segmented worms. The many-jointed Ring-worms (Annelida) in their inner structure are closely allied to the individual arms or radii of the Star-fishes, that is to the original single worms, which each arm represents. Each of the five worms of the Star-fish is a chain composed of a great number of equi-formal members, or metamera, lying one behind the other, like every segmented Worm, and every Arthropod. As in the latter a central nervous cord, the ventral nerve cord runs along the central line of the ventral wall of each segment. On each metameron there is a pair of non-jointed feet, and besides these, in most cases, one or more hard thorns or bristles similar to those of many Ring-worms. A detached arm of a Star-fish can lead an independent life, and can then, by the radially-directed growth of buds at one end, again become a complete star.

The most important proofs, however, of the truth of my hypothesis are furnished by the ontogeny or the individual development of the Echinoderma. The most remarkable facts of this ontogeny were first discovered in the year 1848 by the great zoologist, Johannes Müller of Berlin. Some of its most important stages are represented on Plates VIII. and IX. (Compare their explanation in the Appendix.) Fig. _A_ on Plate IX. shows us a common Sea-star (Uraster), Fig. _B_, a Sea-lily (Comatula), Fig. _C_, a Sea-urchin (Echinus), and Fig. _D_, a Sea-cucumber (Synapta). In spite of the extraordinary difference of form manifested by these four representatives of the different classes of Star-fishes, yet the beginning of their development is identical in all cases. Out of the egg an animal-form develops which is utterly different from the fully developed Star-fish, but very like the ciliated larvæ of certain segmented Worms (Star-worms and Ring-worms). This peculiar animal-form is generally called the “larva,” but more correctly the “nurse” of these Star-fish. It is very small and transparent, swims about by means of a fringe of cilia, and is always composed of two equal symmetrical halves or sides. The fully grown Echinoderm, however—which is frequently more than a hundred times larger, and quite opaque—creeps at the bottom of the sea, and is always composed of at least five co-ordinate pieces, or antimera, in the form of radii. Plate VIII. shows the development of the “nurses” of the four Echinoderms represented on Plate IX.

The fully developed Echinoderm arises by a very remarkable process of budding in the interior of the “nurse,” of which it retains little more than the stomach. The nurse, erroneously called the “larva,” of the Echinoderm, must accordingly be regarded as a solitary worm, which by internal budding produces a second generation, in the form of a stock of star-shaped and connected worms. The whole of this process is a genuine alternation of generations, or metagenesis, not a “metamorphosis,” as is generally though erroneously stated. A similar alternation of generations also occurs in many other worms, especially in some star worms (Sipunculidæ), and cord worms (Nemertinæ). Now if, bearing in mind the fundamental law of biogeny, we refer the ontogeny of Echinoderma to their phylogeny, then the whole historical development of the Star-fishes suddenly becomes clear and intelligible to us, whereas without this hypothesis it remains an insoluble mystery. (Compare Gen. Morph. ii. pp. 95-99.)

Besides the reasons mentioned, there are many other facts (principally from the comparative anatomy of Echinoderma) which most distinctly prove the correctness of my hypothesis. I established this hypothesis in 1866, without having any idea that _fossil articulated worms_ still existed, apparently answering to the hypothetical primary forms. Such have in the mean time, however, really been discovered. In a treatise “On the Equivalent of the North American Taconic Schist in Germany,”[3] Geinitz and Liebe, in 1867, have described a number of articulated Silurian worms, which completely confirm my suppositions. Numbers of these very remarkable worms are found in an excellent state of preservation in the slates of Würzbach, in the upper districts of Reusz. They are of the same structure as the articulated arm of a Star-fish, and evidently possessed a hard coat of mail, a much denser, more solid cutaneous skeleton than other worms in general. The number of body-segments, or metamera, is very considerable, so that the worms, although no more than a quarter or half an inch in breadth, attained a length of from two to three feet. The excellently preserved impressions, especially those of the Phyllodocites thuringiacus and Crossopodia Henrici, are so like the arms of many Star-fish (Colastra) that their true blood relationship seems very probable. This primæval group of worms, which are most probably the ancestors of Star-fish, I call Mailed worms (Phracthelminthes, p. 150.)

The three other classes of Echinoderma evidently arose at a later period out of the class of Sea-stars which have most faithfully retained the original form of the stellate colony of worms. The Sea-lilies, or Crinoida, differ least from them, but having given up the free, slow motion possessed by other Sea-stars, they have become adherent to rocks, etc., and form for themselves a long stalk. Some Encrinites, however (for example, the Comatulæ, Fig. _B_, on Plates VIII. and IX.), afterwards detach themselves from their stalk. The original worm individuals in the Crinoida are indeed no longer preserved in the same independent condition as in the case of the common star-fish; but they nevertheless always possess articulated arms extending from a common central disc. Hence we may unite the Sea-lilies and Sea-stars into a main-class, or branch, characterized as possessing articulated arms (Colobrachia).

In the other two classes of Echinoderma, the Sea-urchins and Sea-cucumbers, the articulated arms are no longer present as independent parts, but, by the increased centralization of the stock, have completely fused so as to form a common, inflated, central disc, which now looks like a simple box or capsule without arms. The original stock of five individuals has apparently degenerated to the form-value of a simple individual, a single person. Hence we may represent these two classes as a branch characterized as being without arms (Lipobrachia), equivalent to those which possess articulated arms. The first of these two classes, that of Sea-urchins (Echinida) takes its name from the numerous and frequently very large thorns which cover the hard shell, which is itself artistically built up of calcareous plates. (Fig. _C_, Plates VIII. and IX.) The fundamental form of the shell itself is a pentagonal pyramid. The Sea-urchins probably developed directly out of the group of Sea-stars. The different classes and orders of marine lilies and stars which are given in the following table, illustrate the laws of progress and differentiation in a striking manner. In each succeeding period of the earth’s history we see the individual classes continually increasing in variety and perfection. (Gen. Morph. ii. Plate IV.)

The history of three of these classes of Star-fish is very minutely recorded by numerous and excellently preserved fossils, but on the other hand, we know almost nothing of the historical development of the fourth class, that of the Sea-cucumbers (Holothuriæ). These curious sausage-shaped Star-fish manifest externally a deceptive similarity to worms. (Fig. _D_, Plates VIII. and IX.) The skeletal structures in their skin are very imperfect, and hence no distinct remains of their elongated, cylindrical, worm-like body could be preserved in a fossil state. However, from the comparative anatomy of the Holothuriæ, we can infer that they have arisen, by the softening of the cutaneous skeleton, from members of the class of Sea-urchins.

From the Star-fish we turn to the fifth and most highly developed tribe of the invertebrate animals, namely, the phylum of Articulata, or those with _jointed feet_ (Arthropoda). As has already been remarked, this tribe corresponds to Linnæus’ class of Insects. It contains four classes: (1) the genuine six-legged Insects, or Flies; (2) the eight-legged Spiders; (3) the Centipedes, with numerous pairs of legs; and (4) the Crabs, or Crustacea, whose legs vary in number. The last class breathe water through gills, and may therefore be contrasted as the main-class of gill-breathing Arthropoda, or Gilled Insects (Carides), with the three first classes. The latter breathe air by means of peculiar wind-pipes, or tracheæ, and may therefore appropriately be united to form the main-class of the trachea-breathing Arthropoda, or Tracheate Insects (Tracheata).

In all animals with articulated feet, as the name indicates, the legs are distinctly articulated, and by this, as well as by the strong differentiation of the separate parts of the body, or metamera, they are sharply distinguished from Ringed worms, with which Bär and Cuvier classed them. They are, however, in every respect so like the Ringed worms that they can scarcely be considered altogether distinct from them. They, like the Ringed worms, possess a very characteristic form of the central nervous system, the so-called ventral marrow, which commences in a gullet-ring encircling the mouth. From other facts also, it is evident that the Arthropoda developed at a late period out of articulated worms. Probably either the Wheel Animalcules or the Ringed worms are their nearest blood relations in the Worm tribe. (Gen. Morph. ii. Plate V. pp. 85-102.)

Now, although the derivation of the Arthropoda from ringed Worms may be considered as certain, still it cannot with equal assurance be maintained that the whole tribe of the former has arisen out of one branch of the latter. For several reasons seem to support the supposition that the Gilled Arthropods have developed out of a branch of articulated worms, different from that which gave rise to the Tracheate Arthropods. But on the whole it remains more probable that both main-classes have arisen out of one and the same group of Worms. In this case the Tracheate Insects—Spiders, Flies, and Centipedes—must have branched off at a later period from the gill-breathing Insects, or Crustacea.

The pedigree of the Arthropoda can on the whole be clearly made out from the palæontology, comparative anatomy, and ontogeny of its four classes, although here, as everywhere else, many details remain very obscure. Not until the history of the individual development of all the different groups has become more accurately known than it is at present, can this obscurity be removed. The history of the class of Gilled Insects, or Crabs (Carides), is at present that best known to us; they are also called encrusted animals (Crustacea), on account of the hard crust or covering of their body. The ontogeny of these animals is extremely interesting and, like that of Vertebrate animals, distinctly reveals the essential outlines of the history of their tribe, that is, their phylogeny. Fritz Müller, in his work, “Für Darwin,”(16) which has already been referred to, has explained this remarkable series of facts in a very able manner.

The common primary form of all Crabs, which in most cases is even now the first to develop out of the egg, is originally one and the same, the so-called _Nauplius_. This remarkable primæval crab represents a very simple form of articulated animal, the body of which in general has the form of a roundish, oval, or pear-shaped disc, and has on its ventral side only three pairs of legs. The first of these is uncloven, the two subsequent pairs are forked. In front, above the mouth, lies a simple, single eye. Although the different orders of the Crustacean class differ very widely from one another in the structure of their body and its appendages, yet the early Nauplius form always remains essentially the same. In order to be convinced of this, let the reader look attentively at Plates X. and XI., a more detailed explanation of which is given in the Appendix. On Plate XI. we see the fully developed representatives of six different orders of Crabs, a Leaf-footed Crab (Limnetis, Fig. _A c_); a Stalked Crab (Lepas, Fig. _D c_); a Root Crab (Sacculina, Fig. _E c_); a Boatman Crab (Cyclops, Fig. _B c_); a Fish Louse (Lernæocera, Fig. _C c_); and, lastly, a highly developed Shrimp (Peneus, Fig. _F c_). These six crabs vary very much, as we see, in the entire form of body, in the number and formation of the legs, etc. When, however, we look at the earliest stages, or “nauplius,” of these six different classes, after they have crept out of the egg—those marked with corresponding letters on Plate X. (Fig. _A n-F n_)—we shall be surprised to find how much they agree. The different forms of Nauplius of these six orders differ no more from one another than would six different “good species” of one genus. Consequently, we may with assurance infer a common derivation of all those orders from a common Primæval Crab, which was essentially like the Nauplius of the present day.

SYSTEMATIC SURVEY

_Of the 7 Legions and 20 Orders of Crabs, or Crustacea._

-------------------------+--------------------------+----------------------+-------------- _Legions of the_ | _Orders of the_ | _Systema Name_ | _Name of a_ _Crustaceæ._ | _Crustaceæ._ | _of the Orders._ | _Genus as an_ | | | _example._ -------------------------+--------------------------+----------------------+-------------- I. ENTOMOSTRACA, _Lower Crustacea_, or Segmented Crabs (not passing through the actual Zoëa form in youth). ------------------------------------------------------------------------------------------

+I. Branchiopoda+ { 1. Primæval Crabs 1. Archicarida Nauplius Gill-footed Crabs { 2. Leaf-foot Crabs 2. Phyllopoda Limnetis { 3. Trilobites 3. Trilobita Paradoxides { 4. Water Fleas 4. Cladocera Daphnia { 5. Bivalve Crabs 5. Ostracoda Cypris

+II. Pectostraca+ { 6. Barnacle Crabs 6. Cirripedia Lepas Fixed Crabs { 7. Root Crabs 7. Rhizocephala Sacculina

+III. Copepoda+ { 8. Boatmen Crabs 8. Eucopepoda Cyclops Oar-footed Crabs { 9. Fish Lice 9. Siphonostoma Lernæocera

+IV. Pantopoda+ {10. No-body Crabs 10. Pycnogonida Nymphon No-body Crabs {

+V. Pœcilopoda+ {11. Spear-tails 11. Xiphosura Limulus Shield Crabs {12. Giant Crabs 12. Gigantostraca Eurypterus

------------------------------------------------------------------------------------------ II. MALACOSTRACA, _Higher Crustacea_, or Mailed Crabs (passing through the Zoëa form in youth). ------------------------------------------------------------------------------------------

+VI. Podophthalma+ {13. Zoëa Crabs 13. Zoëpoda Zoëa Stalk-eyed Mailed {14. Split-legged Crabs 14. Schizopoda Mysis Crabs {15. Mouth-footed Crabs 15. Stomatopoda Squilla {16. Ten-footed Crabs 16. Decapoda Peneus

+VII. Edriophthalma+ {17. Cuma Crabs 17. Cumacea Cuma Mailed Crabs with {18. Flea Crabs 18. Amphipoda Gammarus sessile eyes {19. Wizard Crabs 19. Læmodipoda Caprella {20. Louse Crabs 20. Isopoda Oniscus

----------------------------------------------------------------

(Articulated Worms)

The pedigree on p. 177 will show how we may at present approximately conceive the derivation of the twenty orders of Crustacea enumerated on p. 176, from the common primary form of the Nauplius. Out of the Nauplius form—which originally existed as an independent genus—the five legions of lower Crabs developed as diverging branches in different directions, which in the systematic survey of the class are united as Segmented Crabs (Entomostraca). The higher division of Mailed Crabs (Malacostraca) have likewise originated out of the common Nauplius form. The Nebalia is still a direct form of transition from the Phyllopods to the Schizopods, that is, to the primary form of the stalk-eyed and sessile-eyed Mailed Crabs. The Nauplius at this stage gives rise to another larva form, the so-called Zoëa, which is of great importance. The order of Schizopoda, those with cloven feet (Mysis, etc.), probably originated from this curious Zoëa; they are at present still directly allied, through the Nebalia to the Phyllopoda, those with foliaceous feet. But of all living crabs the Phyllopods are the most closely allied to the original primary form of the Nauplius. Out of the Schizopoda the stalk-eyed and sessile-eyed Mailed Crabs, or Malacostraca, developed as two diverging branches in different directions: the former through shrimps (Peneus, etc.), the latter through the Cumacea (Cuma, etc.), which are still living and closely allied to the Schizopoda. Among those with stalked eyes is the river crab (cray-fish), the lobster, and the others with long tails, or the Macrura, out of which, in the chalk period, the short-tailed crabs, or Brachyura, developed by the degeneration of the tail. Those with sessile eyes divide into the two branches of Flea-crabs (Amphipoda) and Louse-crabs (Isopoda); among the latter are our common Rock-slaters and Wood-lice.

The second main-class of Articulated animals, that of the Tracheata, or air-breathing Tracheate Insects[4] (Spiders, Centipedes, and Flies) did not develop until the beginning of the palæolithic era, after the close of the archilithic period, because all these animals (in contrast with the aquatic crabs) are originally inhabitants of land. It is evident that the Tracheata can have developed only after the lapse of the Silurian period when terrestrial life first began. But as fossil remains of spiders and insects have been found, even in the carboniferous beds, we can pretty accurately determine the time of their origin. The development of the first Tracheate Insects out of gill-bearing Zoëa-crabs, must have taken place between the end of the Silurian and the beginning of the coal period, that is, in the Devonian period.

Gegenbaur, in his excellent “Outlines of Comparative Anatomy,”(21) has lately endeavoured to explain the origin of the Tracheata by an ingenious hypothesis. The system of tracheæ, or air pipes, and the modifications of organization dependent upon it, distinguish Flies, Centipedes, and Spiders so much from other animals, that the conception of its first origin presents no inconsiderable difficulties to phylogeny. According to Gegenbaur, of all living Tracheate Insects, the Primæval Flies, or Archiptera, are most closely allied to the common primary form of the Tracheata. These insects—among which we may especially mention the delicate Day flies (Ephemera), and the agile dragon-flies (Libellula)—in their earliest youth, as larvæ, frequently possess _external tracheate gills_ which lie in two rows on the back of the body, and are shaped like a leaf or paint-brush. Similar leaf or paint-brush shaped organs are met with as real water-breathing organs or gills, in many crabs and ringed worms, and, moreover, in the latter as real dorsal appendages or limbs. The “tracheate gills,” found in the larvæ of many primæval winged insects, must in all probability be explained as “_dorsal limbs_,” and as having developed out of the corresponding appendages of the Annelida, or possibly as having really arisen out of similar parts in Crustacea long since extinct. The present tracheal respiration of the Tracheata developed at a later period out of respiration through the “tracheate gills.” The tracheate gills themselves, however, have in some cases disappeared, and in others become transformed into the _wings_ of the Flies. They have disappeared entirely in the classes of Spiders and Centipedes, and these groups must accordingly be conceived of as degenerated or peculiarly developed lateral branches of the Fly class, which at an early period branched off from the common primary form of Flies; Spiders probably did so at an earlier period than Centipedes. Whether that common primary form of all Tracheata, which in my General Morphology I have named Protracheata, did develop directly out of genuine Ringed worms, or at first out of Crustacea of the Zoëa form (Zoëpoda, p. 177) will probably be settled at some future time by a more accurate knowledge and comparison of the ontogeny of the Tracheata, Crustacea, and Annelida. However, the root of the Tracheata, as well as that of the Crustacea, must in any case be looked for in the group of Ringed worms.

The genuine Spiders (Arachnida) are distinguished from Flies by the absence of wings, and by four pairs of legs; but, as is distinctly seen in the Scorpion-spiders and Tarantulæ, they, like Flies, possess in reality only three pairs of genuine legs. The apparent “fourth pair of legs” in spiders (the foremost) are in reality a pair of feelers. Among the still existing Spiders, there is a small group which is probably very closely allied to the common primary form of the whole class; this is the order of Scorpion-spiders, or Solifugæ, (Solpuga, Galeodes), of which several large species live in Africa and Asia, and are dreaded on account of their poisonous bite. Their body consists—as we suppose to have been the case in the common ancestor of the Tracheata—of a head possessing several pairs of feelers like legs, of a thorax, to the three rings of which are attached three pairs of legs, and of a hinder body, or abdomen, consisting of many distinct rings. In the articulation of their body, the Solifugæ are therefore in reality more closely related to flies than to other spiders. Out of the Devonian Primæval Spiders, which were nearly related to the Solifugæ of the present day, the Long Spiders, the Tailor Spiders, and the Round Spiders probably developed as three diverging branches.

The _Long Spiders_ (Arthrogastres), in which the earlier articulation of body has been better preserved than in Round Spiders, appear to be the older and more original forms. The most important members of this sub-class are the scorpions, which are connected with the Solifugæ through the Tarantella (or Phrynidæ). The small book scorpions, which inhabit our libraries and herbariums, appear as a degenerate lateral branch from the true scorpions. Midway between the Scorpions and Round Spiders are the long-legged Tailor-spiders (Opiliones) which have possibly arisen out of a special branch of the Solifugæ. The Pycnogonida, or No-body Crabs, and the Arctisca, or Bear Worms—still generally included among Long Spiders—must be completely excluded from the class of Spiders; the former belong to the Crustacea, the latter to Ringed worms.

SYSTEMATIC SURVEY

_Of the 3 Classes and 17 Orders of the Tracheata._

------------------+---------------------+----------------------+---------------------- _Classes of the_ | _Sub-Classes of the_| _Order of the_ | _Two Names of_ _Tracheata._ | _Tracheata._ | _Tracheata._ |_Genera as examples._ ------------------+---------------------+----------------------+---------------------- { { 1. Scorpion spiders { Solpuga { { _Solifugæ_ { Galeodes { { { { 2. Tarantella { Phrynus { { _Phrynida_ { Thelyphonus { I. { I. { Long spiders { 3. Scorpions { Scorpio =Spiders= { _Arthrogastres_ { _Scorpioda_ { Buthus { { +Arachnida+ { { 4. Book scorpions { Obisium { { _Pseudoscorpioda_ { Chelifer { { { { 5. Tailor spiders { Phalangium { { _Opilionida_ { Opilio { { { II. { 6. Spinning spiders { Epeira { Round spiders { _Araneæ_ { Mygale { { { { _Sphærogastres_ { 7. Mites { Sarcoptes { { _Acarida_ { Demodex

II. { III. =Centipedes= { Simple-footed { 8. Simple-footed { Scolopendra { _Chilopoda_ { _Chilopoda_ { Geophilus { { +Scolopendria+ { IV. { 9. Double-footed { Julus or { Double-footed { _Diplopoda_ { Polydesmus +Myriapoda+ { _Diplopoda_ {

{ { 10. Primitive flies { Ephemera { { _Archiptera_ { Libellula { { { { 11. Gauze-wings { Hemerobius { { _Neuroptera_ { Phryganea { V. { { Chewing { 12. Straight-wings { Locusta { _Masticantia_ { _Orthoptera_ { Forficula { { III. { { 13. Beetles { Cicindela =Flies= { { _Coleoptera_ { Melolontha { { +Hexapoda+ { { 14. Bee-wings { Apis { { _Hymenoptera_ { Formica { { { { 15. Bugs { Aphis { VI. { _Hemiptera_ { Cimex { Sucking { { _Sugentia_ { 16. Two-wings { Culex { { _Diptera_ { Musca { { { { 17. Butterflies { Bombyx

----------------------------------------------------------------

Fossil remains of Long Spiders are found in the Coal. The second sub-class of the Arachnida, the _Round Spiders_ (Sphærogastres), first appear in the fossil state in the Jura, that is, at a very much later period. They have developed out of a branch of the Solifuga, by the rings of the body becoming more and more united with one another. In the true _Spinning Spiders_ (Araneæ), which we admire on account of their delicate skill in weaving, the union of the joints of the trunk, or metamera, goes so far, that the trunk now consists of only two pieces, of a head-breast (cephalo-thorax) with jaws, feelers, and four pairs of legs, and of a hinder body without appendages, where the spinning warts are placed. In _Mites_ (Acarida), which have probably arisen by degeneration (especially by parasitism) out of a lateral branch of Spinning Spiders, even these two trunk pieces have become united and now form an unsegmented mass.

The class of _Scolopendria_, _Myriapoda_, or Centipedes, the smallest and poorest in forms of the four classes of Arthropoda, is characterized by a very elongated body, like that of a segmented Ringed worm, and often possesses more than a hundred pairs of legs. But these animals also originally developed out of a six-legged form of Tracheata, as is distinctly proved by the individual development of the millipede in the egg. Their embryos have at first only three pairs of legs, like genuine insects, and only at a later period do the posterior pairs of legs bud, one by one, from the growing rings of the hinder body. Of the two orders of Centipedes (which in our country live under barks of trees, in moss, etc.) the round, _double-footed_ ones (Diplopoda) probably did not develop until a later period out of the older flat, _single-footed_ ones (Chilopoda), by successive pairs of rings of the body uniting together. Fossil remains of the Chilopoda are first met with in the Jura period.

The third and last class of the Arthropoda breathing through tracheæ, is that of the _Flies_, or _Insects_, in the narrow sense of the word (Insecta, or Hexapoda), the largest of all classes of animals, and next to that of Mammalia, also the most important. Although Flies develop a greater variety of genera and species than all other animals taken together, yet these are all in reality only superficial variations of a single type, which is entirely and constantly preserved in its essential characteristics. In all Flies the three divisions of the trunk—head, breast (thorax), and hinder body are quite distinct. The _hinder body_, or _abdomen_, as in the case of spiders, has no articulated appendages. The central division, the _breast_ or _thorax_, has on its ventral side three pairs of legs, on its back _two pairs of wings_. It is true that, in very many Flies, one or both pairs of wings have become reduced in size or have even entirely disappeared; but the comparative anatomy of Flies distinctly shows that this deficiency has arisen only gradually by the degeneration of the wings, and that all the Flies existing at present are derived from a common, primary Fly, which possessed three pairs of legs and two pairs of wings. (Compare p. 256.) These wings, which so strikingly distinguish Flies from all other Arthropoda, probably arose, as has been already shown, out of the tracheate gills which may still be observed in the larvæ of the ephemeral flies (Ephemera) which live in water.

The head of Flies universally possesses, besides the eyes, a pair of articulated feelers, or antennæ, and also three jaws upon each side of the mouth. These _three pairs of jaws_, although they have arisen in all Flies from the same original basis, by different kinds of adaptation, have become changed to very varied and remarkable forms in the various orders, and are therefore employed for distinguishing and characterizing the main divisions of the class. In the first place, we may distinguish two main divisions, namely, Flies with _chewing_ mandibles (Masticantia) and Flies with _sucking_ mouths (Sugentia). On a closer examination each of these two divisions may again be divided into two sub-groups. Among chewing Flies, or Masticantia, we may distinguish the biting and the licking ones. _Biting flies_ (Mordentia) comprise the most ancient and primæval winged Flies, the gauzy-winged (Neuroptera), straight-winged (Orthoptera), and beetles (Coleoptera). _Licking flies_ (Lambentia) are represented by the one order of skin-winged (Hymenoptera) Flies. We distinguish two groups of _Sucking Flies_, or Sugentia, namely, those which prick and those which sip. There are two orders of pricking Flies (Pungentia), those with half wings (Hemiptera) and gnats and blow-flies (Diptera); butterflies are the only _sipping_ Flies (Sorbentia), Lepidoptera.

Biting Flies, and indeed the order of _Primæval Flies_ (Archiptera, or Pseudoneuroptera) are nearest akin to the still living Flies, and include the most ancient of all Flies, the primary forms of the whole class (hence also those of all Tracheata). Among them are, first of all, the Ephemeral Flies (Ephemera) whose larvæ which live in water, in all probability still show us in their tracheæ-gills the organs out of which the wings of Flies were originally developed. This order further contains the well known dragon-flies, or Libellula, the wine-glass sugar mites (Lepisma), the hopping Flies with bladder-like feet (Physopoda), and the dreaded Termites, fossil remains of which are found even in coal. The order of Gauze-winged Flies (Neuroptera), probably developed directly out of the primæval Flies, which differ from them only by their perfect series of transformations. Among them are the gauze-flies (Planipennia), caddis-flies (Phryganida), and fan-flies (Strepsiptera). Fossil Flies, which form the transition from the primæval Flies (Libellula) to the gauze-winged (Sialidæ), are found even in coal (Dictyophylebia).

The order of _Straight-winged Flies_ (Orthoptera) developed at an early period out of another branch of the primæval Flies by differentiation of the two pairs of wings. This division is composed of one group with a great variety of forms—cockroaches, grasshoppers, crickets, etc. (Ulonata)—and of a smaller group consisting only of the well-known earwigs (Labidura), which are characterised by nippers at the hinder end of their bodies. Fossil remains of cockroaches, as well as of crickets and grasshoppers, have been found in coal.

Fossil remains of the fourth order of Biting Flies, _beetles_ (Coleoptera) likewise occur in coal. This extremely comprehensive order—the favourite one of amateurs and collectors—shows more clearly than any other what infinite variety of forms can be developed externally by adaptation to different conditions of life, without the internal structure and the original form of the body being in any way essentially changed. Beetles have probably developed out of a branch of the straight-winged Flies, from which they differ only in their transformations (larva, pupa, etc.).

The one order of _Licking Flies_, namely, the interesting group of the _Bees_, or _Skin-winged Flies_ (Hymenoptera), is closely allied to the four orders of biting Flies. Among them are those Flies which have risen to such an astonishing degree of mental development, of intellectual perfection, and strength of character, by their extensive division of labour, formation of communities and states, and surpass in this not merely most invertebrate animals, but even most animals in general. This may be said especially of all ants and bees, also of wasps, leaf-wasps, wood-wasps, gall-wasps, etc. They are first met with in a fossil state in the oolites, but they do not appear in greater numbers until the tertiary period. Probably these insects developed either out of a branch of the primæval Flies or the gauze-winged Flies.

Of the two orders of _Pricking Flies_ (Hemiptera and Diptera), that containing the _Half-winged Flies_ (Hemiptera), also called Beaked Flies (Rhynchota), is the older of the two. It includes three sub-orders, viz., the leaf-lice (Homoptera), the bugs (Heteroptera), and lice (Pediculina). Fossil remains of the first two classes are found in the oolites; but an ancient Fly (Eugereon) is found in the Permian system, and seems to indicate the derivation of the Hemiptera from the Neuroptera. Probably the most ancient of the three sub-orders of the Hemiptera are the Homoptera, among which, besides the actual leaf-lice, are the shield-lice, leaf-fleas, and leaf-crickets, or Cicadæ. Lice have probably developed out of two different branches of Homoptera, by continued degeneration (especially by the loss of wings); bugs, on the other hand, by the perfecting and differentiation of the two pairs of wings.

The second order of _pricking flies_, namely, the _Two-winged Flies_ (Diptera), are also found in a fossil state in the oolites, together with Half-winged Flies; but they probably developed out of the Hemiptera by the degeneration of the hind wings. In Diptera the fore wings alone have remained perfect. The principal portion of this order consists of the elongated gnats (Nemocera) and of the compact blow-flies and house-flies (Brachycera), the former of which are probably the older of the two. However, remains of both are found in the oolitic period. The two small groups of lice-flies (Pupipara) forming chrysales, and the hopping-fleas (Aphaniptera), probably developed out of the Diptera by degeneration resulting from parasitism.

The eighth and last order of Flies, and at the same time the only one with mouth-parts adapted to sipping liquids, consists of _moths_ and _butterflies_ (Lepidoptera). This order appears, in several morphological respects, to be the most perfect class of Flies, and accordingly was the last to develop. For we only know of fossil remains of this order from the tertiary period, whereas the three preceding orders extend back to the oolites, and the four biting orders even to the coal period. The close relationship between some moths (Tineæ) and (Noctuæ), and some caddis-flies (Phryganida) renders it probable that butterflies have developed from this group, that is, out of the order of Gauze-winged Flies, or Neuroptera.

The whole history of Flies, and, moreover, the history of the whole tribe of Arthropoda, essentially confirms the great laws of differentiation and perfecting which, according to Darwin’s theory of selection, must be considered as the necessary results of Natural Selection. The whole tribe, so rich in forms, begins in the Archilithic period with the class of _Crabs_ breathing by gills, and with the lowest _Primæval Crabs_, or Archicaridæ. The form of these Primæval Crabs, which were developed out of segmented worms, is still approximately preserved by the remarkable _Nauplius_, in the common larval stage of so many Crabs. Out of the Nauplius, at a later period, the curious Zoëa was developed, which is the common larval form of all the higher or mailed crabs (Malacostraca), and, at the same time, possibly of that Arthopod which at first breathed through tracheæ, and became the common ancestor of all _Tracheata_. This Devonian ancestor, which must have originated between the end of the Silurian and the beginning of the Coal period, was probably most closely related to the still living Primæval Flies, or _Archiptera_. Out of these there developed, as the main tribe of the Tracheata, the class of Flies, from the lowest stage of which the _spiders_ and _centipedes_ separated as two diverging branches. Throughout a long period there existed only the four biting orders of Flies—the Primæval flies, Gauze-wings, Straight-wings, and the Beetles, the first of which is probably the common primary form of the three others. It was only at a much later period that the Licking, Pricking, and Sipping flies developed out of the Biting ones, which retained the original form of the three pairs of jaws most distinctly. The following table will show once more how these orders succeeded one another in the history of the earth.

CLASSIFICATION OF FLIES.

{ { 1. Primæval winged { M.I. } { { _Archiptera_ { A.A. } { { } { I. { 2. Gauze-winged { M.C. } A. { Biting Flies { _Neuroptera_ { A.A. } { { } =Flies= { _Mordentia_ { 3. Straight-winged { M.I. } =with Chewing= { { _Orthoptera_ { A.D. } =Mouths= { { } { { 4. Beetles { M.C. } +Masticantia+ { { _Coleoptera_ { A.D. } { { { II. { 5. Skin-winged { M.C. } { Licking Flies { _Hymenoptera_ { A.A. } { _Lambentia_ { } } } { { 6. Half-winged { M.I. } { III. { _Hemiptera_ { A.A. } B. { Stinging Flies { } =Flies= { { 7. Tway-flies { M.C. } =with Sucking= { _Pungentia_ { _Diptera_ { A.D. } =Mouths= { { +Sugentia+ { IV. { 8. Butterflies { M.C. } { Sipping Flies { _Lepidoptera_ { A.A. } { { { } { _Sorbentia_ { { }

_Note._—The difference in the metamorphosis or transformation and in the development of the wings of the eight individual orders of Flies is also specified by the following letters: M.I. = Imperfect Metamorphosis. M.C. = Perfect Metamorphosis. (Compare Gen. Morph. ii. p. 99.) A.A. = Equal wings (fore and hinder wings are the same, or differ but little). A.D. = Unequal wings (fore and hinder wings very different in structure and texture, occasioned by strong differentiation).