CHAPTER VI.

THE DISTRIBUTION AND THE ÆTIOLOGY OF THE CRAYFISHES.

So far as I have been able to discover, all the crayfishes which inhabit the British islands agree in every point with the full description given above, at p. 230. They are abundant in some of our rivers, such as the Isis, and other affluents of the Thames; and they have been observed in those of Devon;[15] but they appear to be absent from many others. I cannot hear of any, for example, in the Cam or the Ouse, on the east, or in the rivers of Lancashire and Cheshire, on the west. It is still more remarkable that, according to the best information I can obtain, they are absent in the Severn, though they are plentiful in the Thames and Severn canal. Dr. M^cIntosh, who has paid particular attention to the fauna of Scotland, assures me that crayfish are unknown north of the Tweed. In Ireland, on the other hand, they occur in many localities;[16] but the question whether their diffusion, and even their introduction into this {289} island, has or has not been effected by artificial means, is involved in some obscurity.

[15] Moore. Magazine of Natural History. New Series, III., 1839.

[16] Thompson. Annals and Magazine of Natural History, XI., 1843.

English zoologists have always termed our crayfish _Astacus fluviatilis_; and, up to a recent period, the majority of Continental naturalists have included a corresponding form of _Astacus_ under that specific name.

Thus M. Milne Edwards, in his classical work on the _Crustacea_,[17] published in 1837, observes under the head of “Écrevisse commune. _Astacus fluviatilis_:” “There are two varieties of this crayfish; in the one, the rostrum gradually becomes narrower from its base onwards, and the lateral spines are situated close to its extremity; in the other, the lateral edges of the rostrum are parallel in their posterior half and the lateral spines are stronger and more remote from the end.”

The “first variety,” here mentioned, is known under the name of “Écrevisse à pieds blancs”[18] in France, by way of distinction from the “second variety,” which is termed “Écrevisse à pieds rouges,” on account of the more or less extensive red coloration of the forceps and ambulatory limbs. This second variety is the larger, commonly attaining five inches in length, and sometimes reaching much larger dimensions; and it is more highly esteemed for the market, on account of its better flavour.

[17] “Histoire Naturelle des Crustacés.”

[18] Carbonnier. “L’Écrevisse,” p. 8.

In Germany, the two forms have long been popularly distinguished, the former by the name of “Steinkrebs,” {290} or “stone crayfish,” and the latter by that of “Edelkrebs,” or “noble crayfish.”

Milne Edwards, it will be observed, speaks of these two forms of crayfish as “varieties” of the species _Astacus fluviatilis_; but, even as far back as the year 1803 some zoologists began to regard the “stone crayfish” as a distinct species, to which Schrank applied the name of _Astacus torrentium_, while the “noble crayfish” remained in possession of the old denomination, _Astacus fluviatilis_; and, subsequently, various forms of “stone-crayfishes” have been further distinguished as the species _Astacus saxatilis_, _A. tristis_, _A. pallipes_, _A. fontinalis_, &c. On the other hand, Dr. Gerstfeldt,[19] who has devoted especial attention to the question, denies that these are anything more than varieties of one species; but he holds this and Milne Edwards’s “second variety” to be specifically distinct from one another.

We thus find ourselves in the presence of three views respecting the English and French crayfishes.

1. They are all varieties of one species—_A. fluviatilis_.

2. There are two species—_A. fluviatilis_, and _A. torrentium_, of which last there are several varieties.

3. There are, at fewest, five or six distinct species.

Before adopting the one or the other of these views, it is necessary to form a definite conception of the meaning of the terms “species” and “variety.” {291}

[19] “Ueber die Flusskrebse Europas.” Mém. de l’Acad. de St. Petersburg, 1859.

The word “species” in Biology has two significations; the one based upon morphological, the other upon physiological considerations.

A species, in the strictly morphological sense, is simply an assemblage of individuals which agree with one another, and differ from the rest of the living world in the sum of their morphological characters; that is to say, in the structure and in the development of both sexes. If the sum of these characters in one group is represented by A, and that in another by A + _n_; the two are morphological species, whether _n_ represents an important or an unimportant difference.

The great majority of species described in works on Systematic Zoology are merely morphological species. That is to say, one or more specimens of a kind of animal having been obtained, these specimens have been found to differ from any previously known by the character or characters _n_; and this difference constitutes the definition of the new species, and is all we really know about its distinctness.

But, in practice, the formation of specific groups is more or less qualified by considerations based upon what is known respecting variation. It is a matter of observation that progeny are never exactly like their parents, but present small and inconstant differences from them. Hence, when specific identity is predicated of a group of individuals, the meaning conveyed is not that they are all exactly alike, but only that their differences are so {292} small, and so inconstant, that they lie within the probable limits of individual variation.

Observation further acquaints us with the fact, that, sometimes, an individual member of a species may exhibit a more or less marked variation, which is propagated through all the offspring of that individual, and may even become intensified in them. And, in this manner, a _variety_, or _race_, is generated within the species; which variety, or race, if nothing were known respecting its origin, might have every claim to be regarded as a separate morphological species. The distinctive characters, of a race, however, are rarely equally well marked in all the members of the race. Thus suppose the species A to develop the race A + _x_; then the difference _x_ is apt to be much less in some individuals than in others; so that, in a large suite of specimens, the interval between A + _x_ and A will be filled up by a series of forms in which _x_ gradually diminishes.

Finally, it is a matter of observation that modification of the physical conditions under which a species lives favours the development of varieties and races.

Hence, in the case of two specimens having respectively the characters A and A + _n_, although, _primâ facie_, they are of distinct species; yet if a large collection shows us that the interval between A and A + _n_ is filled up by forms of A having traces of _n_, and forms of A + _n_ in which _n_ becomes less and less, then it will be {293} concluded that A and A + _n_ are races of one species and not separate species. And this conclusion will be fortified if A and A + _n_ occupy different stations in the same geographical area.

Even when no transitional forms between A and A + _n_ are discoverable, if _n_ is a small and unimportant difference, such as of average size, colour, or ornamentation, it may be fairly held that A and A + _n_ are mere varieties; inasmuch as experience proves that such variations may take place comparatively suddenly; or the intermediate forms may have died out and thus the evidence of variation may have been effaced.

From what has been said it follows that the groups termed morphological species are provisional arrangements, expressive simply of the present state of our knowledge.

We call two groups species, if we know of no transitional forms between them, and if there is no reason to believe that the differences which they present are such as may arise in the ordinary course of variation. But it is impossible to say whether the progress of inquiry into the characters of any group of individuals may prove that what have hitherto been taken for mere varieties are distinct morphological species; or whether, on the contrary, it may prove that what have hitherto been regarded as distinct morphological species are mere varieties.

What has happened in the case of the crayfish is this: {294} the older observers lumped all the Western European forms which came under their notice under one species, _Astacus fluviatilis_; noting, more or less distinctly, the stone crayfish and the noble crayfish as races or varieties of that species. Later zoologists, comparing crayfishes together more critically, and finding that the stone crayfish is ordinarily markedly different from the noble crayfish, concluded that there were no transitional forms, and made the former into a distinct species, tacitly assuming that the differential characters are not such as could be produced by variation.

It is at present an open question whether further investigation will or will not bear out either of these assumptions. If large series of specimens of both stone crayfishes and noble crayfishes from different localities are carefully examined, they will be found to present great variations in size and colour, in the tuberculation of the carapace and limbs, and in the absolute and relative sizes of the forceps.

The most constant characters of the stone crayfish are:—

1. The tapering form of the rostrum and the approximation of the lateral spines to its point; the distance between these spines being about equal to their distance from the apex of the rostrum (fig. 61, A).

2. The development of one or two spines from the ventral margin of the rostrum.

3. The gradual subsidence of the posterior part of {295} the post-orbital ridge, and the absence of spines on its surface.

4. The large relative size of the posterior division of the telson (G).

On the contrary, in the noble crayfish:—

1. The sides of the posterior two-thirds of the rostrum are nearly parallel, and the lateral spines are fully a third of the length of the rostrum from its point; the distance between them being much less than their distance from the apex of the rostrum (B).

2. No spine is developed from the ventral margin of the rostrum.

3. The posterior part of the post-orbital ridge is a more or less distinct, sometimes spinous elevation.

4. The posterior division of the telson is smaller relatively to the anterior division (H).

I may add that I have found three rudimentary pleurobranchiæ in the noble crayfish, and never more than two in the stone crayfish.

In order to ascertain whether no crayfish exist in which the characters of the parts here referred to are intermediate between those defined, it would be necessary to examine numerous examples of each kind of crayfish from all parts of the areas which they respectively inhabit. This has been done to some extent, but by no means thoroughly; and I think that all that can be safely said, at present, is that the existence of intermediate forms is not proven. But, whatever the constancy of the {296} differences between the two kinds of crayfishes, there can surely be no doubt as to their insignificance; and no question that they are no more than such as, judging by analogy, might be produced by variation.

From a morphological point of view, then, it is really impossible to decide the question whether the stone crayfish and the noble crayfish should be regarded as species or as varieties. But, since it will, hereafter, be convenient to have distinct names for the two kinds, I shall speak of them as _Astacus torrentium_ and _Astacus nobilis_.[20]

[20] According to strict zoological usage the names should be written _A. fluviatilis_ (var. _torrentium_) and _A. fluviatilis_ (var. _nobilis_) on the hypothesis that the stone crayfish and the noble crayfish are varieties; and _A. torrentium_ and _A. fluviatilis_ on the hypothesis that they are species; but as I neither wish to prejudge the species question, nor to employ cumbrously long names, I take a third course.

In the physiological sense, a species means, firstly, a group of animals the members of which are capable of completely fertile union with one another, but not with the members of any other group; and, secondly, it means all the descendants of a primitive ancestor or ancestors, supposed to have originated otherwise than by ordinary generation.

It is clear that, even if crayfishes had an unbegotten ancestor, there is no means of knowing whether the stone crayfish and the noble crayfish are descendants of the same, or of different ancestors, so that the second sense of species hardly concerns us. As to the first sense, there is no evidence to show whether the two {297} kinds of crayfish under consideration are capable of fertile union or whether they are sterile. It is said, however, that hybrids or mongrels are not met with in the waters which are inhabited by both kinds, and that the breeding season of the stone crayfish begins earlier than that of the noble crayfish.

M. Carbonnier, who practises crayfish culture on a large scale, gives some interesting facts bearing on this question in the work already cited. He says that, in the streams of France, there are two very distinct kinds of crayfishes—the red-clawed crayfish (L’Écrevisse à pieds rouges), and the white-clawed crayfish (L’Écrevisse à pieds blancs), and that the latter inhabit the swifter streams. In a piece of land converted into a crayfish farm, in which the white-clawed crayfish existed naturally in great abundance, 300,000 red-clawed crayfish were introduced in the course of five years; nevertheless, at the end of this time, no intermediate forms were to be seen, and the “pieds rouges” exhibited a marked superiority in size over the “pieds blancs.” M. Carbonnier, in fact, says that they were nearly twice as big.

On the whole, the facts as at present known, seem to incline rather in favour of the conclusion that _A. torrentium_ and _A. nobilis_ are distinct species; in the sense that transitional forms have not been clearly made out, and that, possibly, they do not interbreed.

* * * * *

As I have already remarked, the very numerous {298} specimens of English and Irish crayfishes which have passed through my hands, have all presented the character of _Astacus torrentium_, with which also the description given in works of recognised authority coincides as far as it goes.[21] The same form is found in many parts of France, as far south as the Pyrenees, and it is met with as far east as Alsace and Switzerland. I have recently[22] been enabled, by the kindness of Dr. Bolivar, of Madrid, who sent me a number of crayfishes from the neighbourhood of that city, to satisfy myself that the Spanish peninsula contains crayfishes altogether similar to those of Britain, except that the subrostral spine is less developed. Further, I have no doubt that Dr. Heller[23] is right in his identification of the English crayfish with a form which he describes under the name of _A. saxatilis_. He says that it is especially abundant in Southern Europe, and that it occurs in Greece, in Dalmatia, in the islands of Cherso and Veglia, at Trieste, in the Lago di Garda, and at Genoa. Further, _Astacus torrentium_ appears to be widely distributed in North Germany. The eastern limit of this crayfish is uncertain; but, according to Kessler,[24] it does not occur within the limits of the Russian empire. {299}

[21] See Bell. “British Stalk-eyed Crustacea,” p. 237.

[22] Since the statement respecting the occurrence of crayfishes in Spain on p. 44 was printed.

[23] “Die Crustaceen des Südlichen Europas,” 1863.

[24] “Die Russischen Flusskrebse.” Bulletin de la Société Impériale des Naturalistes de Moscow, 1874.

_Astacus torrentium_ appears to be particularly addicted to rapid highland streams and the turbid pools which they feed.

_Astacus nobilis_ is indigenous to France, Germany, and the Italian peninsula. It is said to be found at Nice and at Barcelona, though I cannot hear of it elsewhere in Spain. Its south-eastern limit appears to be the Lake of Zirknitz, in Carniola, not far from the famous caves of Adelsberg. It is not known in Dalmatia, in Turkey, nor in Greece. In the Russian empire, according to Kessler, this crayfish chiefly inhabits the watershed of the Baltic. The northern limit of its distribution lies between Christianstad, in the Gulf of Bothnia (62° 16′ N), and Serdobol, at the northern end of Lake Ladoga. “Eastward of Lake Ladoga it is found in the Uslanka, a tributary of the Swir. It appears to be the only crayfish which exists in the waters which flow from the south into the Gulf of Finland and into the Baltic; except in those streams and lakes which have been artificially connected with the Volga, and in which it is partially replaced by _A. leptodactylus_.” It still inhabits the Lakes of Beresai and Bologoe, as well as the affluents of the Msta and the Wolchow; and it is met with in affluents of the Dnieper, as far as Mohilew. _Astacus nobilis_ is also found in Denmark and Southern Sweden; but, in the latter country, its introduction appears to have been artificial. This crayfish is said occasionally to be met with on the Livonian coast in the waters of the Baltic, which, however, it must {300} be remembered, are much less salt than ordinary sea water.

It will be observed that while the two forms, _A. torrentium_ and _A. nobilis_, are intermixed over a large part of Central Europe, _A. torrentium_ has a wider north-westward, south-westward, and south-eastward extension, being the sole occupant of Britain, and apparently of the greater part of Spain and of Greece. On the other hand, in the northern and eastern parts of Central Europe, _A. nobilis_ appears to exist alone.

Further to the east, a new form, _Astacus leptodactylus_ (fig. 75), makes its appearance. Whether _A. leptodactylus_ exists in the upper waters of the Danube, does not appear, but in the lower Danube and in the Theiss it is the dominant, if not the exclusive, crayfish. From hence it extends through all the rivers which flow into the Black, Azov, and Caspian Seas, from Bessarabia and Podolia on the west, to the Ural mountains on the east. In fact, the natural habitat of this crayfish appears to be the watershed of the Pontocaspian area, excluding that part of the Black Sea which lies southward of the Caucasus on the one hand, and of the mouths of the Danube on the other.[25]

[25] These statements rest on the authority of Kessler and Gerstfeldt, in their memoirs already cited.

It is a remarkable circumstance that this crayfish not only thrives in the brackish waters of the estuaries of the rivers which debouche into the Black Sea and the Sea of Azov, but that it is found even in the salter {302} southern parts of the Caspian, in which it lives at considerable depths.

In the north, _Astacus leptodactylus_ is met with in the rivers which flow into the White Sea, as well as in many streams and lakes about the Gulf of Finland. But it has probably been introduced into these streams by the canals which have been constructed to connect the basin of the Volga with the rivers which flow into the Baltic and into the White Sea. In the latter, the invading _A. leptodactylus_ is everywhere overcoming and driving out _A. nobilis_ in the struggle for existence, apparently in virtue of its more rapid multiplication.[26]

[26] Kessler (Die Russischen Flusskrebse, l. c. p. 369–70), has an interesting discussion of this question.

In the Caspian and in the brackish waters of the estuaries of the Dniester and the Bug, a somewhat different crayfish, which has been called _Astacus pachypus_, occurs; another closely allied form (_A. angulosus_) is met with in the mountain streams of the Crimea and of the northern face of the Caucasus; and a third, _A. colchicus_, has recently been discovered in the Rion, or Phasis of the ancients, which flows into the eastern extremity of the Black Sea.

With respect to the question whether these Pontocaspian crayfishes are specifically distinct from one another, and whether the most widely distributed kind, _A. leptodactylus_, is distinct from _A. nobilis_, exactly the same difficulties arise as in the case of the west European {303} crayfishes. Gerstfeldt, who has had the opportunity of examining large series of specimens, concludes that the Pontocaspian crayfishes and _A. nobilis_ are all varieties of one species. Kessler, on the contrary, while he admits that _A. angulosus_ is, and _A. pachypus_ may be, a variety of _A. leptodactylus_, affirms that the latter is specifically distinct from _A. nobilis_.

Undoubtedly, well marked examples of _A. leptodactylus_ are very different from _A. nobilis_.

1. The edges of the rostrum are produced into five or six sharp spines, instead of being smooth or slightly serrated as in _A. nobilis_.

2. The fore part of the rostrum has no serrated spinous median keel, such as commonly, though not universally, exists in _A. nobilis_.

3. The posterior end of the post-orbital ridge is still more distinct and spiniform than in _A. nobilis_.

4. The abdominal pleura of _A. leptodactylus_ are narrower, more equal sided, and triangular in shape.

5. The chelæ of the forceps, especially in the males, are more elongated; and the moveable and fixed claws are slenderer and have their opposed edges straighter and less tuberculated.

But, in all these respects, individual specimens of _A. nobilis_ vary in the direction of _A. leptodactylus_ and _vice versâ_; and if _A. angulosus_ and _A. pachypus_ are varieties of _A. leptodactylus_, I cannot see why Gerstfeldt’s conclusion that _A. nobilis_ is another variety of {304} the same form need be questioned on morphological grounds. However, Kessler asserts that, in those localities in which _A. leptodactylus_ and _A. nobilis_ live together, no intermediate forms occur, which is presumptive evidence that they do not intermix by breeding.

* * * * *

No crayfishes are known to inhabit the rivers of the northern Asiatic watershed, such as the Obi, Yenisei, and Lena. None are known[27] in the sea of Aral, or the great rivers Oxus and Jaxartes, which feed that vast lake; nor any in the lakes of Balkash and Baikal. If further exploration verifies this negative fact, it will be not a little remarkable; inasmuch as two[28], if not more, kinds of crayfishes are found in the basin of the great river Amur, which drains a large area of north-eastern Asia, and debouches into the Gulf of Tartary, in about the latitude of York.

Japan has one species (_A. japonicus_), perhaps more; but no crayfish has as yet been made known in any part of eastern Asia, south of Amurland. There are certainly none in Hindostan; none are known in Persia, Arabia, or Syria. In Asia Minor the only recorded locality is the Rion. No crayfish has yet been discovered in the whole continent of Africa.[29] {305}

[27] It would be hazardous, however, to assume that none exist, especially in the Oxus, which formerly flowed into the Caspian.

[28] _A. dauricus_ and _A. Schrenckii_.

[29] Whatever the so-called _Astacus capensis_ of the Cape Colony may be, it is certainly not a crayfish.

Thus, on the continent of the old world, the crayfishes are restricted to a zone, the southern limit of which coincides with certain great geographical features; on the west, the Mediterranean, with its continuation, the Black Sea; then the range of the Caucasus, followed by the great Asiatic highlands, as far as the Corea on the east. On the north, though there is no such physical boundary, the crayfishes appear to be entirely excluded from the Siberian river basins; while east and west, though a sea-barrier exists, the crayfishes extend beyond it, to reach the British islands and those of Japan.

Crossing the Pacific, we meet with some half-a-dozen kinds of crayfishes,[30] different from those of the old world, but still belonging to the genus _Astacus_, in British Columbia, Oregon, and California. Beyond the Rocky Mountains, from the Great Lakes to Guatemala, crayfishes abound, as many as thirty-two different species having been described, but they all belong to the genus _Cambarus_ (fig. 63, p. 248). Species of this genus also occur in Cuba,[31] but, so far as is at present known, not in any of the other West Indian islands. The occurrence of a curious dimorphism among the male _Cambari_ has been described by Dr. Hagen; and a blind _Cambarus_ {306} is found, along with other blind animals, in the subterranean caves of Kentucky.

[30] Dr. Hagen in his “Monograph of the North American Astacidæ,” enumerates six species; _A. Gambelii_, _A. klamathensis_, _A. leenisculus_, _A. nigrescens_, _A. oreganus_, _and A. Trowbridgii_.

[31] Von Martens. _Cambarus cubensis._ Archiv. für Naturgeschichte, xxxviii.

All the crayfishes of the northern hemisphere belong to the _Potamobiidæ_, and no members of this family are known to exist south of the equator. The crayfishes of the southern hemisphere, in fact, all belong to the division of the _Parastacidæ_, and in respect of the number and variety of forms and the size which they reach, the head-quarters of the _Parastacidæ_ is the continent of Australia. Some of the Australian crayfishes (fig. 76) attain a foot or more in length, and are as large as full-sized lobsters. The genus _Engæus_ of Tasmania comprises small crayfish which, like some of the _Cambari_, live habitually on land, in burrows which they excavate in the soil.

New Zealand has a peculiar genus of crayfishes, _Paranephrops_, a species of which is found in the Fiji Islands, but none are known to occur elsewhere in Polynesia.

Two kinds of crayfish have been obtained in southern Brazil, and have been described by Dr. v. Martens,[32] as _A. pilimanus_ and _A. brasiliensis_. I have shown that they belong to a peculiar genus, _Parastacus_. The former was procured at Porto Alegre, which is situated in 30° S. Latitude, close to the mouth of the Jacuhy, at the north end of the great Laguna do Patos, which {308} communicates by a narrow passage with the sea; and also at Sta. Cruz in the upper basin of the Rio Pardo, an affluent of the Jacuhy, “by digging it out of holes in the ground.” The latter (_P. brasiliensis_, fig. 64) was obtained at Porto Alegre, and further inland, in the region of the primitive forest at Rodersburg, in shallow streams.

[32] Südbrasilische Süss- und Brackwasser Crustaceen, nach den Sammlungen des Dr. Reinh. Hensel. Archiv. für Naturgeschichte, XXXV. 1869.

[33] The nomenclature of the Australian crayfishes requires thorough revision. I therefore, for the present, assign no name to this crayfish. It is probably identical with the _A. nobilis_ of Dana and the _A. armatus_ of Von Martens.

In addition to these, no crayfish have as yet been found in any of the great rivers, such as the Orinoko; the Amazon, in which they were specially sought for by Agassiz; or in the La Plata, on the eastern side of the Andes. But, on the west, an “_Astacus_” _chilensis_ is described in the “Histoire Naturelle des Crustacées,” (vol. ii. p. 333). It is here stated that this crayfish “habite les côtes du Chili,” but the freshwaters of the Chilian coast are doubtless to be understood.

Finally, Madagascar has a genus and species of crayfish (_Astacoides madagascariensis_, fig. 65) peculiar to itself.

* * * * *

On comparing the results obtained by the study of the geographical distribution of the crayfishes with those brought to light by the examination of their morphological characters, the important fact that there is a broad and general correspondence between the two becomes apparent. The wide equatorial belt of the earth’s surface which separates the crayfishes of the northern from those of the southern hemisphere, is a sort of geographical {310} representation of the broad morphological differences which mark off the _Potamobiidæ_ from the _Parastacidæ_. Each group occupies a definite area of the earth’s surface, and the two are separated by an extensive border-land untenanted by crayfishes.

A similar correspondence is exhibited, though less distinctly, when we consider the distribution of the genera and species of each group. Thus, among the _Potamobiidæ_, _Astacus torrentium_ and _nobilis_ belong essentially to the northern, western, and southern watersheds of the central European highlands, the streams of which flow respectively into the Baltic and the North Seas, the Atlantic and the Mediterranean (fig. 77, I.); _A. leptodactylus_, _pachypus_, _angulosus_, and _colchicus_, appertain to the Pontocaspian watershed, the rivers of which drain into the Black Sea and the Caspian (I.); while _Astacus dauricus_ and _A. Schrenckii_ are restricted to the widely separated basin of the Amur, which sheds its waters into the Pacific (II.) The _Astaci_ of the rivers of western North America, which flow into the Pacific (IV.), and the _Cambari_ of the Eastern or Atlantic water-shed (V.) are separated by the great physical barrier of the Rocky Mountain ranges. Finally, with regard to the _Parastacidæ_, the widely separated geographical regions of New Zealand (VIII.), Australia (IX.), Madagascar (XII.), and South America (VI. and VII.), are inhabited by generically distinct groups.

But when we look more closely into the matter, it will {311} be found that the parallel between the geographical and the morphological facts cannot be quite strictly carried out.

_Astacus torrentium_, as we have seen, inhabits both the British Islands and the continent of Europe; nevertheless, there is every reason to believe that twenty miles of sea water is an insuperable barrier to the passage of crayfishes from one land to the other. For though some crayfishes live in brackish water, there is no evidence that any existing species can maintain themselves in the sea. A fact of the same character meets us at the other side of the Eurasiatic continent, the Japanese and the Amurland crayfishes being closely allied; although it is not clear that there are any identical species on the two sides of the Sea of Japan.

Another circumstance is still more remarkable. The West American crayfishes are but little more different from the Pontocaspian crayfishes, than these are from _Astacus torrentium_. On the face of the matter, one might therefore expect the Amurland and Japanese crayfishes, which are intermediate in geographical position, to be also intermediate, morphologically, between the Pontocaspian and the West American forms. But this is not the case. The branchial system of the Amurland _Astaci_ appears to be the same as that of the rest of the genus; but, in the males, the third joint (ischiopodite) of the second and third pair of ambulatory limbs is provided with a conical, recurved, hook-like process; while, in the females, the hinder edge of the penultimate thoracic {312} sternum is elevated into a transverse prominence, on the posterior face of which there is a pit or depression.[34]

In both these characters, but more especially in the former, the Amurland and Japanese _Astaci_ depart from both the Pontocaspian and the West American _Astaci_, and approach the _Cambari_ of Eastern North America.

In these crayfishes, in fact, one or both of the same pairs of legs in the male are provided with similar hook-like processes; while, in the females, the modification of the penultimate thoracic sternum is carried still further and gives rise to the curious structure described by Dr. Hagen as the “annulus ventralis.”

[34] Kessler, l. c.

In all the _Cambari_, the pleurobranchiæ appear to be entirely suppressed, and the hindermost podobranchia has no lamina; while the areola is usually extremely narrow. The proportional size of the areola in the Amurland {313} crayfishes is not recorded; in the Japanese crayfish, judging by the figure given by De Haan, it is about the same as in the western _Astaci_. On the other hand, in the West American crayfishes it is distinctly smaller; so that, in this respect, they perhaps more nearly approach the _Cambari_. Unfortunately, nothing is known as to the branchiæ of the Amurland crayfishes. According to De Haan, those of the Japanese species resemble those of the western _Astaci_: as those of the West American _Astaci_ certainly do.

With respect to the _Parastacidcæ_; in the remarkable length and flatness of the epistoma, the crayfishes of Australia, Madagascar, and South America, resemble one another. But in its peculiar truncated rostrum (see fig. 65) and in the extreme modification of its branchial system, which I have described elsewhere, the Madagascar genus stands alone.

The _Paranephrops_ of New Zealand and the Fijis, with its wide and short epistoma, long rostrum, and large antennary squames, is much more unlike the Australian forms than might be expected from its geographical position. On the other hand, considering their wide separation by sea, the amount of resemblance between the New Zealand and the Fiji species is very remarkable.

* * * * *

If the distribution of the crayfishes is compared with that of terrestrial animals in general, the points of {314} difference are at least as remarkable as the resemblances.

With respect to the latter, the area occupied by the _Potamobiidæ_, corresponds roughly with the Palæarctic and Nearctic divisions of the great Arctogæal provinces of distribution indicated by mammals and birds; while distinct groups of crayfishes occupy a larger or smaller part of the other, namely, the Austro-Columbian, Australian, and Novozelanian primary distributional provinces of mammals and birds. Again, the peculiar crayfishes of Madagascar answer to the special features of the rest of the fauna of that island.

But the North American crayfishes extend much further South than the limits of the Nearctic fauna in general; while the absence of any group of crayfishes in Africa, or in the rest of the old world, south of the great Asiatic table-land, forms a strong contrast to the general resemblance of the North African and Indian fauna to that of the rest of Arctogæa. Again, there is no such vast difference between the crayfishes of New Zealand, Australia, and South America, as there is between the mammals and the birds of those regions.

It may be concluded, therefore, that the conditions which have determined the distribution of crayfishes have been very different from those which have governed the distribution of mammals and birds. But if we compare with the distribution of the crayfishes, not that of terrestrial animals in general, but only that of freshwater {315} fishes, some very curious points of approximation become manifest. The _Salmonidæ_, or fishes of the salmon and trout kind, a few of which are exclusively marine, many both marine and freshwater, while others are confined to fresh water, are distributed over the northern hemisphere, in a manner which recalls the distribution of the Potamobine crayfishes,[35] though they do not extend so far to the South in the new world, while they go a little further, namely, as far as Algeria, Northern Asia Minor, and Armenia, in the old world. With the exception of the single genus _Retropinna_, which inhabits New Zealand, no true salmonoid fish occurs south of the equator; but, as Dr. Günther has pointed out, two groups of freshwater fishes, the _Haplochitonidæ_ and the _Galaxidæ_, which stand in somewhat the same relation to the _Salmonidæ_ as the _Parastacidæ_ do to the _Potamobiidæ_, take the place of the _Salmonidæ_ in the fresh waters of New Zealand, Australia, and South America. There are two species of _Haplochiton_ in Tierra del Fuego; and of the closely allied genus _Prototroctes_, one species is found in South Australia, and one in New Zealand; of the _Galaxidæ_, the same species, _Galaxias attennuatus_, occurs in the streams of New Zealand, Tasmania, the Falkland Islands, and Peru.

[35] According to Dr. Günther their southern range is similarly limited by the Asiatic Highlands. But they abound in the rivers both of the old and new worlds which flow into the Arctic sea; and though those on the western side of the Rocky Mountains are different from the Eastern American forms, yet there are species common to both the Asiatic and the American coasts of the North Pacific.

Thus, these fish avoid South Africa, as the crayfishes {316} do; but I am not aware that any member of the group is found in Madagascar, and thus completes the analogy.

* * * * *

The preservation of the soft parts of animals in the fossil state depends upon favourable conditions of rare occurrence; and, in the case of the _Crustacea_, it is not often that one can hope to meet with such small hard parts as the abdominal members, in a good state of preservation. But without recourse to the branchial apparatus, and to the abdominal appendages, it might be very difficult to say whether a given crustacean belonged to the Astacine, or to the closely allied Homarine group. Of course, if the accompanying fossils indicated that the deposit in which the remains occur, was of freshwater origin, the presumption in favour of their Astacine nature would be very strong; but if they were inhabitants of the sea, the problem whether the crustacean in question was a marine Astacine, or a true Homarine, might be very hard to solve.

Undoubted remains of crayfishes have hitherto been discovered only in freshwater strata of late tertiary age. In Idaho, North America, Professor Cope[36] found, in association with _Mastodon mirificus_, and _Equus excelsus_, several species, which he considers to be distinct from {317} the existing American crayfishes; whether they are _Cambari_ or _Astaci_ does not appear. But, in the lower chalk of Ochtrup, in Westphalia, and therefore in a marine deposit, Von der Marck and Schlüter[37] have obtained a single, somewhat imperfect, specimen of a crustacean, which they term _Astacus politus_, and which, singularly enough, has the divided telson found only in the genus _Astacus_. It would be very desirable to know more about this interesting fossil. For the present it affords a strong presumption that a marine Potamobine existed as far back as the earlier part of the cretaceous epoch.

[36] On three extinct _Astaci_ from the freshwater Tertiary of Idaho. Proceedings of the American Philosophical Society, 1869–70.

[37] Neue Fische und Krebse aus der Kreide von Westphalen. Palæontographica, Bd. XV., p. 302; tab. XLIV., figs. 4 and 5.

* * * * *

Such are the more important facts of Morphology, Physiology, and Distribution, which make up the sum of our present knowledge of the Biology of Crayfishes. The imperfection of that knowledge, especially as regards the relations between Morphology and Distribution, becomes a serious drawback when we attack the final problem of Biology, which is to find out why animals of such structure and active powers, and so localized, exist?

It would appear difficult to frame more than two fundamental hypotheses in attempting to solve this problem. Either we must seek the origin of crayfishes in conditions extraneous to the ordinary course of natural {318} operations, by what is commonly termed Creation; or we must seek for it in conditions afforded by the usual course of nature, when the hypothesis assumes some shape of the doctrine of Evolution. And there are two forms of the latter hypothesis; for, it may be assumed, on the one hand, that crayfishes have come into existence, independently of any other form of living matter, which is the hypothesis of spontaneous or equivocal generation, or abiogenesis; or, on the other hand, we may suppose that crayfishes have resulted from the modification of some other form of living matter; and this is what, to borrow a useful word from the French language, is known as _transformism_.

I do not think that any hypothesis respecting the origin of crayfishes can be suggested, which is not referable to one or other of these, or to a combination of them.

As regards the hypothesis of creation, little need be said. From a scientific point of view, the adoption of this speculation is the same thing as an admission that the problem is not susceptible of solution. Moreover, the proposition that a given thing has been created, whether true or false, is not capable of proof. By the nature of the case direct evidence of the fact is not obtainable. The only indirect evidence is such as amounts to proof that natural agencies are incompetent to cause the existence of the thing in question. But such evidence is out of our reach. The most that {319} can be proved, in any case, is that no known natural cause is competent to produce a given effect; and it is an obvious blunder to confound the demonstration of our own ignorance with a proof of the impotence of natural causes. However, apart from the philosophical worthlessness of the hypothesis of creation, it would be a waste of time to discuss a view which no one upholds. And, unless I am greatly mistaken, at the present day, no one possessed of knowledge sufficient to give his opinion importance is prepared to maintain that the ancestors of the various species of crayfish were fabricated out of inorganic matter, or brought from nothingness into being, by a creative fiat.

Our only refuge, therefore, appears to be the hypothesis of evolution. And, with respect to the doctrine of abiogenesis, we may also, in view of a proper economy of labour, postpone its discussion until such time as the smallest fragment of evidence that a crayfish can be evolved by natural agencies from not living matter, is brought forward.

In the meanwhile, the hypothesis of transformism remains in possession of the field; and the only profitable inquiry is, how far are the facts susceptible of interpretation, on the hypothesis that all the existing kinds of crayfish are the product of the metamorphosis of other forms of living beings; and that the biological phenomena which they exhibit are the results of the interaction, through past time, of two series of {320} factors: the one, a process of morphological and concomitant physiological modification; the other, a process of change in the condition of the earth’s surface.

If we set aside, as not worth serious consideration, the assumption that the _Astacus torrentium_ of Britain was originally created apart from the _Astacus torrentium_ of the Continent; it follows, either that this crayfish has passed across the sea by voluntary or involuntary migration; or that the _Astacus torrentium_ existed before the English Channel, and spread into England while these islands were still continuous with the European mainland; and that the present isolation of the English crayfishes from the members of the same species on the Continent is to be accounted for by those changes in the physical geography of western Europe which, as there is abundant evidence to prove, have separated the British Islands from the mainland.

There is no evidence that our crayfish has been purposely introduced by human agency into Great Britain; and from the mode of life of crayfish and the manner in which the eggs are carried about by the parent during their development, transport by birds or floating timber would seem to be out of the question. Again, although _Astacus nobilis_ is said to venture into the brackish waters of the Gulf of Finland, and _A. leptodactylus_, as we have seen, makes itself at home in the more or less salt Caspian, there is no reason to believe that _Astacus torrentium_ is capable of existing in {321} sea-water, still less of crossing the many miles of sea which separate England from even the nearest point of the Continent. In fact, the existence of the same kind of crayfish on both sides of the Channel appears to be only a case of the general truth, that the Fauna of the British Islands is identical with a part of that of the Continent; and as our foxes, badgers, and moles certainly have neither swum across, nor been transported by man, but existed in Britain while it was still continuous with western Europe, and have been isolated by the subsequent intervention of the sea, so we may confidently explain the presence of _Astacus torrentium_ by reference to the same operation.

If we take into account the occurrence of _Astacus nobilis_ over so large a part of the area occupied by _Astacus torrentium_; its absence in the British Islands, and in Greece; and the closer affinity which exists between _A. nobilis_ and _A. leptodactylus_, than between _A. nobilis_ and _A. torrentium_; it seems not improbable that Astacus torrentium was the original tenant of the whole western European area outside the Ponto-Caspian watershed; and that _A. nobilis_ is an invading offshoot of the Ponto-Caspian or _leptodactylus_ form which has made its way into the western rivers in the course of the many changes of level which central Europe has undergone; in the same way as _A. leptodactylus_ is now passing into the rivers of the Baltic provinces of Russia.

The study of the glacial phenomena of central Europe {322} has led Sartorius von Waltershausen[38] to the conclusion that at the time when the glaciers of the Alps had a much greater extension than at present, a vast mass of freshwater extended from the valley of the Danube to that of the Rhone, around the northern escarpment of the Alpine chain, and connected the head-waters of the Danube with those of the Rhine, the Rhone, and the northern Italian rivers. As the Danube debouches into the Black Sea, and this was formerly connected with the Aralo-Caspian Sea, an easy passage would thus be opened up by which crayfishes might pass from the Aralo-Caspian area to western Europe. If they spread by this road, the _Astacus torrentium_ may represent the first wave of migration westward, while _A. nobilis_ answers to a second, and _A. leptodactylus_, with its varieties, remains as the representative of the old Aralo-Caspian crayfishes. And thus the crayfishes would present a curious parallel with the Iberian, Aryan, and Mongoloid streams of westward movement among mankind.

If we thus suppose the western Eurasiatic crayfishes to be simply varieties of a primitive Aralo-Caspian stock, their limitation to the south by the Mediterranean and by the great Asiatic highlands becomes easily intelligible.

[38] “Untersuchungen ueber die Klimate der Gegenwart und der Vorwelt.” Natuurkundige Verhandelingen van de Hollandsche Maatschappij der Wetenschappen te Haarlem, 1865.

The extremely severe climatal conditions which obtain in northern Siberia may sufficiently account for the {323} absence of crayfishes (if they are really absent) in the rivers Obi, Yenisei, and Lena, and in the great lake Baikal, which lies more than 1,300 feet above the sea, and is frozen over from November to May. Moreover, there can be no doubt that, at a comparatively recent period, the whole of this region, from the Baltic to the mouth of the Lena, was submerged beneath a southward extension of the waters of the Arctic ocean to the Aralo-Caspian Sea and Lake Baikal, and a westward extension to the Gulf of Finland.

The great lakes and inland seas which stretch, at intervals, from Baikal, on the east, to Wenner in Sweden, on the west, are simply pools, isolated partly by the rising of the ancient sea-bottom and partly by evaporation; and often completely converted into fresh water by the inflow of the surrounding land-drainage. But the population of these pools was originally the same as that of the Northern Ocean, and a few species of marine crustaceans, mollusks, and fish, besides seals, remain in them as living evidences of the great change which has taken place. The same process which, as we shall see, has isolated the _Mysis_ of the Arctic seas in the lakes of Sweden and Finland, has shut up with it other arctic marine crustacea, such as species of _Gammarus_ and _Idothea_. And the very same species of _Gammarus_ is imprisoned, along with arctic seals, in the waters of Lake Baikal.

The distribution of the American crayfishes agrees equally well with the hypothesis of the northern origin of {324} the stock from which they have been evolved. Even under existing geographical conditions, an affluent of the Mississippi, the St. Peter’s river, communicates directly, in rainy weather, with the Red river, which flows into Lake Winnipeg, the southernmost of the long series of intercommunicating lakes and streams, which occupy the low and flat water-parting between the southern and the northern watersheds of the North American Continent. But the northernmost of these, the Great Slave Lake, empties itself by the Mackenzie river into the Arctic Ocean, and thus provides a route by which crayfishes might spread from the north over all parts of North America east of the Rocky Mountains.

The so-called Rocky Mountain range is, in reality, an immense table-land, the edges of which are fringed by two principal lines of mountainous elevations. The table-land itself occupies the place of a great north and south depression which, in the cretaceous epoch, was occupied by the sea and probably communicated with the ocean at its northern, as well as at its southern end. During and since this epoch it became gradually filled up, and it now contains an immense thickness of deposits of all ages from the cretaceous to the pliocene—the earlier marine, the later more and more completely freshwater. During the tertiary epoch, various portions of this area have been occupied by vast lakes, the more northern of which doubtless had outlets into the Northern sea. That crayfish existed in the vicinity of the Rocky Mountains {325} in the latter part of the tertiary epoch is testified by the Idaho fossils. And there is thus no difficulty in understanding their presence in the rivers which have now cut their way to the Pacific coast.

The similarity of the crayfish of the Amurland and of Japan is a fact of the same order as the identity of the English crayfish with the _Astacus torrentium_ of the European Continent, and is to be explained in an analogous fashion. For there can be no doubt that the Asiatic continent formerly extended much further to the eastward than it does at present, and included what are now the islands of Japan. Even with this alteration of the geographical conditions, however, it is not easy to see how crayfishes can have got into the Amur-Japanese fresh waters. For a north-eastern prolongation of the Asiatic highlands, which ends to the north in the Stanovoi range, shuts in the Amur basin on the west; while the Amur debouches into the sea of Okhotsk, and the Pacific ocean washes the shores of the Japanese islands.

But there are many grounds for the conclusion that, in the latter half of the tertiary epoch, eastern Asia and North America were connected, and that the chain of the Kurile and Aleutian islands may indicate the position of a great extent of submerged land. In that case, the sea of Okhotsk and Behring’s sea may occupy the site of inland waters which formerly placed the mouth of the Amur in direct communication with the Northern Ocean, just as the Black Sea, at present, brings the basin of the {326} Danube into connection, first with the Mediterranean and then with the western Atlantic; and, as in former times, it gave access from the south to the vast area now drained by the Volga. When the Black Sea communicated with the Aralo-Caspian sea, and this opened to the north into the Arctic sea, a chain of great inland waters must have skirted the eastern frontier of Europe, just such as would now lie on the eastern frontier of Asia if the present coast underwent elevation.

Supposing, however, that the ancestral forms of the _Potamobiidæ_ obtained access to the river basins in which they are now found, from the north, the hypothesis that a mass of fresh water once occupied a great part of the region which is now Siberia and the Arctic Ocean, would be hardly tenable, and it is, in fact, wholly unnecessary for our present purpose.

The vast majority of the stalk-eyed crustaceans are, and always have been, exclusively marine animals; the crayfishes, the _Atyidæ_, and the fluviatile crabs (_Thelphusidæ_), being the only considerable groups among them which habitually confine themselves to fresh waters. But even in such a genus as _Penæus_, most of the species of which are exclusively marine, some, such as _Penæus brasiliensis_, ascend rivers for long distances. Moreover, there are cases in which it cannot be doubted that the descendants of marine _Crustacea_ have gradually accustomed themselves to fresh water conditions, and have, at the same time, become more or less modified, {327} so that they are no longer absolutely identical with those descendants of their ancestors which have continued to live in the sea.[39]

In several of the lakes of Norway, Sweden and Finland, and in Lake Ladoga, in Northern Europe; in Lake Superior and Lake Michigan, in North America; a small crustacean, _Mysis relicta_, occurs in such abundance as to furnish a great part of the supply of food to the fresh water fishes which inhabit these lakes. Now, this _Mysis relicta_ is hardly distinguishable from the _Mysis oculata_ which inhabits the Arctic seas, and is certainly nothing but a slight variety of that species.

In the case of the lakes of Norway and Sweden, there is independent evidence that they formerly communicated with the Baltic, and were, in fact, fiords or arms of the sea. The communication of these fiords with the sea having been gradually cut off, the marine animals they contained have been imprisoned; and as the water has been slowly changed from salt to fresh by the drainage of the surrounding land, only those which were able to withstand the altered conditions have survived. Among these is the _Mysis oculata_, which has in the meanwhile undergone the slight variation which has converted it into _Mysis relicta_. Whether the same explanation {328} applies to Lakes Superior and Michigan, or whether the _Mysis oculata_ has not passed into these masses of fresh water by channels of communication with the Arctic Ocean which no longer exist, is a secondary question. The fact remains that _Mysis relicta_ is a primitively marine animal which has become completely adapted to fresh-water life.

[39] See on this interesting subject: Martens, “On the occurrence of marine animal forms in fresh water.” Annals of Natural History, 1858: Lovèn. “Ueber einige im Wetter und Wener See gefundene Crustaceen.” Halle Zeitschrift für die Gesammten Wissenschaften, xix., 1862: G. O. Sars, “Histoire Naturelle des Crustacés d’eau douce de Norvège,” 1867.

Several species of prawns (_Palæmon_) abound in our own seas. Other marine prawns are found on the coasts of North America, in the Mediterranean, in the South Atlantic and Indian Oceans, and in the Pacific as far south as New Zealand. But species of the same genus (_Palæmon_) are met with, living altogether in fresh water, in Lake Erie, in the rivers of Florida, in the Ohio, in the rivers of the Gulf of Mexico, of the West India Islands and of eastern South America, as far as southern Brazil, if not further; in those of Chili and those of Costa Rica in western South America; in the Upper Nile, in West Africa, in Natal, in the Islands of Johanna, Mauritius, and Bourbon, in the Ganges, in the Molucca and Philippine Islands, and probably elsewhere.

Many of these fluviatile prawns differ from the marine species not only in their great size (some attaining a foot or more in length), but still more remarkably in the vast development of the fifth pair of thoracic appendages. These are always larger than the slender fourth pair (which answer to the forceps of the crayfishes); and, in the males especially, they are very long and strong, and {329} are terminated by great chelæ, not unlike those of the crayfishes. Hence these fluviatile prawns (known in many places by the name of “Cammarons”) are not unfrequently confounded with true crayfishes; though the fact that there are only three pair of ordinary legs behind the largest, forceps-like pair, is sufficient at once to distinguish them from any of the _Astacidæ_.

Species of these large-clawed prawns live in the {330} brackish water lagoons of the Gulf of Mexico, but I am not aware that any of them have yet been met with in the sea itself. The _Palæmon lacustris_ (_Anchistia migratoria_, Heller) abounds in fresh-water ditches and canals between Padua and Venice, and in the Lago di Garda, as well as in the brooks of Dalmatia; but its occurrence in the Adriatic or the Mediterranean, which has been asserted, appears to be doubtful. So the Nile prawn, though very similar to some Mediterranean prawns, does not seem to be identical with any at present known.[40]

In all these cases, it appears reasonable to apply the analogy of the _Mysis relicta_, and to suppose that the fluviatile prawns are simply the result of the adaptive modification of species which, like their congeners, were primitively marine.

[40] Heller, “Die Crustaceen des südlichen Europas,” p. 259. Klunzinger, “Ueber eine Süsswasser-crustacee im Nil,” with the notes by von Martens and von Siebold: Zeitschrift für Wissenschaftliche Zoologie, 1866.

But if the existing sea prawns were to die out, or to be beaten in the struggle for existence, we should have, scattered over the world in isolated river basins, more or less distinct species of freshwater prawns,[41] the areas inhabited by which might hereafter be indefinitely enlarged or diminished, by alteration in the elevation of the {331} land and by other changes in physical geography. And, indeed, under these circumstances, the freshwater prawns themselves might become so much modified, that, even if the descendants of their ancestors remained unchanged in structure and habits in the sea, the relationship of the two might no longer be obvious.

[41] This seems actually to have happened in the case of the widely-spread allies and companions of the fluviatile prawns, _Atya_ and _Caridina_. I am not aware that truly marine species of these genera are known.

These considerations appear to me to indicate the direction in which we must look for a rational explanation of the origin of crayfishes and their present distribution.

I have no doubt that they are derived from ancestors which lived altogether in the sea, as the great majority of the _Mysidæ_ and many of the prawns do now; and that, of these ancestral crayfishes, there were some which, like _Mysis oculata_ or _Penæus brasiliensis_, readily adapted themselves to fresh water conditions, ascended rivers, and took possession of lakes. These, more or less modified, have given rise to the existing crayfishes, while the primitive stock would seem to have vanished. At any rate, at the present time, no marine crustacean with the characters of the _Astacidæ_ is known.

As crayfishes have been found in the later tertiaries of North America, we shall hardly err in dating the existence of these marine crayfishes at least as far back as the miocene epoch; and I am disposed to think that, during the earlier tertiary and later mesozoic periods, these _Crustacea_ not only had as wide a distribution as the Prawns and _Penæi_ have now, but were differentiated into two groups, one with the general characters of the {332} _Potamobiidæ_ in the northern hemisphere, and another, with those of the _Parastacidæ_, in the southern hemisphere.

The ancestral Potamobine form probably presented the peculiarities of the _Potamobiidæ_ in a less marked degree than any existing species does. Probably the four pleurobranchiæ were all equally well developed; the laminæ of the podobranchiæ smaller and less distinct from the stem; the first and second abdominal appendages less specialised; and the telson less distinctly divided. So far as the type was less specially Potamobine, it must have approached the common form in which _Homarus_ and _Nephrops_ originated. And it is to be remarked that these also are exclusively confined to the northern hemisphere.

The wide range and close affinity of the genera _Astacus_ and _Cambarus_ appear to me to necessitate the supposition that they are derived from some one already specialised Potamobine form; and I have already mentioned the grounds upon which I am disposed to believe that this ancestral Potamobine existed in the sea which lay north of the miocene continent in the northern hemisphere.

In the marine primitive crayfishes south of the equator, the branchial apparatus appears to have suffered less modification, while the suppression of the first abdominal appendages, in both sexes, has its analogue among the _Palinuridæ_, the headquarters of which are in the southern hemisphere. That they should have ascended {333} the rivers of New Zealand, Australia, Madagascar, and South America, and become fresh water _Parastacidæ_, is an assumption which is justified by the analogy of the fresh-water prawns. It remains to be seen whether marine _Parastacidæ_ still remain in the South Pacific and Atlantic Oceans, or whether they have become extinct.

* * * * *

In speculating upon the causes of an effect which is the product of several co-operating factors, the nature of each of which has to be divined by reasoning backwards from its effects, the probability of falling into error is very great. And this probability is enhanced when, as in the present case, the effect in question consists of a multitude of phenomena of structure and distribution about which much is yet imperfectly known. Hence the preceding discussion must rather be regarded as an illustration of the sort of argumentation by which a completely satisfactory theory of the ætiology of the crayfish will some day be established, than as sufficing to construct such a theory. It must be admitted that it does not account for the whole of the positive facts which have been ascertained; and that it requires supplementing, in order to furnish even a plausible explanation of various negative facts.

The positive fact which presents a difficulty is the closer resemblance between the Amur-Japanese crayfish and the East American _Cambari_, than between the {334} latter and the West American _Astaci_; and the closer resemblance between the latter and the Pontocaspian crayfish, than either bear to the Amur-Japanese form. If the facts had been the other way, and the West American and Amur-Japanese crayfish had changed places, the case would have been intelligible enough. The primitive Potamobine stock might then have been supposed to have differentiated itself into a western astacoid, and an eastern cambaroid form;[42] the latter would have ascended the American, and the former the Asiatic rivers. As the matter stands, I do not see that any plausible explanation can be offered without recourse to suppositions respecting a former more direct communication between the mouth of the Amur, and that of the North American rivers, in favour of which no definite evidence can be offered at present.

The most important negative fact which remains to be accounted for is the absence of crayfishes in the rivers of a large moiety of the continental lands, and in numerous islands. Differences of climatal conditions are obviously inadequate to account for the absence of crayfishes in Jamaica, when they are present in Cuba; for their absence in Mozambique, and the islands of Johanna and Mauritius, when they are present in Madagascar; and for their absence in the Nile, when they exist in Guatemala. {335}

[42] Just as there is an American form of _Idothea_ and an Asiatic form in the Arctic ocean at the present day.

At present, I confess that I do not see my way to a perfectly satisfactory explanation of the absence of crayfishes in so many parts of the world in which they mighty _à priori_, be expected to exist; and I can only suggest the directions in which an explanation may be sought.

The first of these is the existence of physical obstacles to the spread of crayfishes, at the time at which the Potamobine and the Parastacine stocks respectively began to take possession of the rivers, some of which have now ceased to exist; and the second is the probability that, in many rivers which have been accessible to crayfishes, the ground was already held by more powerful competitors.

If the ancestors of the Potamobine crayfishes originated only among those primitive crayfishes which inhabited the seas north of the miocene continent, their present limitation to the south, in the old world, is as easily intelligible as is their extension southward, in the course of the river basins of Northern America as far as Guatemala, but no further. For the elevation of the Eurasiatic highlands had commenced in the miocene epoch, while the isthmus of Panama was interrupted by the sea.

With respect to the Southern hemisphere, the absence of crayfishes in Mauritius and in the islands of the Indian Ocean, though they occur in Madagascar, may be due to the fact that the former islands are of comparatively late volcanic origin; while Madagascar is the remnant of {336} a very ancient continental area, the oldest indigenous population of which, in all probability, is directly descended from that which occupied it at the beginning of the tertiary epoch. If Parastacine _Crustacea_ inhabited the southern hemisphere at this period, and subsequently became extinct as marine animals, their preservation in the freshwaters of Australia, New Zealand, and the older portions of South America may be understood. The difficulty of the absence of crayfishes in South Africa[43] remains; and all that can be said is, that it is a difficulty of the same nature as that which confronts us when we compare the fauna of South Africa in general with that of Madagascar. The population of the latter region has a more ancient aspect than that of the former; and it may be that South Africa, in its present shape, is of very much later date than Madagascar.

[43] But it must be remembered that we have as yet everything to learn respecting the fauna of the great inland lakes and river systems of South Africa.

With respect to the second point for consideration, it is to be remarked that, in the temperate regions of the world, the crayfishes are by far the largest and strongest of any of the inhabitants of freshwater, except the _Vertebrata_; and that while frogs and the like fall an easy prey to them, they must be formidable enemies and competitors even to fishes, aquatic reptiles, and the smaller aquatic mammals. In warm climates, however, not only the large prawns which have been mentioned, but _Atyæ_ {337} and fluviatile crabs (_Thelphusa_) compete for the possession of the freshwaters; and it is not improbable that under some circumstances, they may be more than a match for crayfishes; so that the latter might either be driven out of territory they already occupied, as _Astacus leptodactylus_ is driving out _A. nobilis_ in the Russian rivers; or might be prevented from entering rivers already tenanted by their rivals.

In connection with this speculation, it is worthy of remark that the area occupied by the fluviatile crabs is very nearly the same as that zone of the earth’s surface from which crayfish are excluded, or in which they are scanty. That is to say, they are found in the hotter parts of the eastern side of the two Americas, the West Indies, Africa, Madagascar, Southern Italy, Turkey and Greece, Hindostan, Burmah, China, Japan, and the Sandwich Islands. The large-clawed fluviatile prawns are found in the same regions of America, on both east and west coasts, in Africa, Southern Asia, the Moluccas, and the Philippine Islands; while the _Atyidæ_ not only cover the same area, but reach Japan, extend over Polynesia, to the Sandwich Islands, on the north, and New Zealand, on the south, and are found on both shores of the Mediterranean; a blind form (_Troglocaris Schmidtii_), in the Adelsberg caves, representing the blind _Cambarus_ of the caves of Kentucky.

* * * * *

The hypothesis respecting the origin of crayfishes {338} which has been tentatively put forward in the preceding pages, involves the assumption that marine Crustacea of the astacine type were in existence during the deposition of the middle tertiary formations, when the great continents began to assume their present shape. That such was the case there can be no doubt, inasmuch as abundant remains of Crustacea of that type occur still earlier in the mesozoic rocks. They prove the existence of ancient crustaceans, from which the crayfishes may have been derived, at that period of the earth’s history when the conformation of the land and sea were such as to admit of their entering the regions in which we now find them.

The materials which have, up to the present, time been collected are too scanty to permit of the tracing out of all the details of the genealogy of the crayfish. Nevertheless, the evidence which exists is perfectly clear, as far as it goes, and is in complete accordance with the requirements of the doctrine of evolution.

Mention has been made of the close affinity between the crayfishes and the lobsters—the _Astacina_ and the _Homarina_; and it fortunately happens that these two groups, which may be included under the common name of the _Astacomorpha_, are readily distinguishable from all the other _Podophthalmia_ by peculiarities of their exoskeleton which are readily seen in all well-preserved fossils. In all, as in the crayfish, there are large forceps, followed by two pairs of chelate ambulatory limbs, while {339} the succeeding two pairs of legs are terminated by simple claws. The exopodite of the last abdominal appendage is divided into two parts by a transverse suture. The pleura of the second abdominal somite are larger than the others, and overlap those of the first somite, which are very small. Any fossil crustacean which presents all these characters, is certainly one of the _Astacomorpha_.

The _Astacina_, again, are distinguished from the _Homarina_ by the mobility of the last thoracic somite, and the characters of the first and second abdominal appendages, when they are present; or by their entire absence. But it is so difficult to make out anything about either of these characters in fossils, that, so far as I am aware, we know nothing about them in any fossil Astacomorph. And hence, it may be impossible to say to which division any given form belongs, unless its resemblances to known types are so minute and so close as to remove doubt.

For the present purpose, the series of the fossiliferous rocks may be grouped as follows:—1. Recent and Quaternary. 2. Newer Tertiary (Pliocene and Miocene). 3. Older Tertiary (Eocene). 4. Cretaceous (Chalk, Greensand and Gault). 5. Wealden. 6. Jurassic (Purbeck to Inferior Oolite). 7. Liassic. 8. Triassic. 9. Permian. 10. Carboniferous. 11. Devonian. 12. Silurian. 13. Cambrian.

Now the oldest known member of the group of the {341} decapod _Podophthalmia_ to which the _Astacomorpha_ belong occurs in the Carboniferous formation. It is the genus _Anthrapalæmon_—a small and very curious crustacean, about which nothing more need be said at present, as it does not appear to have special affinities with the _Astacomorpha_. In the later formations, up to the top of the Trias, podophthalmatous _Crustacea_ are very rare; and, unless the Triassic genus _Pemphix_ is an exception, no Astacomorphs are known to occur in them. The specimens of _Pemphix_ which I have examined are not sufficiently complete to enable me to express any opinion about them.

The case is altered when we reach the Middle Lias. In fact this yields several forms of a genus, _Eryma_ (fig. 80, B), which also occurs in the overlying strata almost up to the top of the Jurassic series, and presents so many variations that nearly forty different species have been recognised. _Eryma_ is, in all respects, an Astacomorph, and so far as can be seen, it differs from the existing genera only in such respects as those in which they differ from one another. Thus it is quite certain that Astacomorphous _Crustacea_ have existed since a period so remote as the older part of the Mesozoic period; and any hesitation in admitting this singular persistency of type on the part of the crayfishes, is at once removed by the consideration of the fact that, along with _Eryma_, in the Middle Lias, prawn-like _Crustacea_, generically identical with the existing _Penæus_, flourished in the sea {342} and left their remains in the mud of the ancient sea bottom.

_Eryma_ is the only crustacean, which can be certainly ascribed to the _Astacomorpha_, that has hitherto been found in the strata from the Middle Lias to the lithographic slates; which last lie in the upper part of the Jurassic series. In the freshwater beds of the Wealden, no _Astacomorpha_ are known, and although no very great weight is to be attached to a negative fact of this kind, it is, so far, evidence that the _Astacomorpha_ had not yet taken to freshwater life. In the marine deposits of the Cretaceous epoch, however, astacomorphous forms, which {343} are known by the generic names of _Hoploparia_ and _Enoploclytia_, are abundant.

The differences between these two genera, and between both and _Eryma_, are altogether insignificant from a broad morphological point of view. They appear to me to be of less importance than those which obtain between the different existing genera of crayfishes.

_Hoploparia_ is found in the London clay. It therefore extends beyond the bounds of the Mesozoic epoch into the older Tertiary. But when this genus is compared with the existing _Homarus_ and _Nephrops_, it is found partly to resemble the one and partly the other. Thus, on one line, the actual series of forms which have succeeded one another from the Liassic epoch to the present day, is such as must have existed if the common lobster and the Norway lobster are the descendants of _Erymoid_ crustaceans which inhabited the seas of the Liassic epoch.

Side by side with _Eryma_, in the lithographic slates, there is a genus, _Pseudastacus_ (fig. 80, A), which, as its name implies, has an extraordinarily close resemblance to the crayfishes of the present day. Indeed there is no point of any importance in which (in the absence of any knowledge of the abdominal appendages in the males) it differs from them. On the other hand, in some features, as in the structure of the carapace, it differs from _Eryma_, much as the existing crayfishes differ from _Nephrops_. Thus, in the latter part of the Jurassic epoch, the Astacine type {344} was already distinct from the Homarine type, though both were marine; and, since _Eryma_ begins at least as early as the Middle Lias, it is possible that _Pseudastacus_ goes back as far, and that the common protastacine form is to be sought in the Trias. _Pseudastacus_ is found in the marine cretaceous rocks of the Lebanon, but has not yet been traced into the Tertiary formations.

I am disposed to think that _Pseudastacus_ is comparable to such a form as _Astacus nigrescens_ rather than to any of the _Parastacidæ_, as I doubt the existence of the latter group at any time in northern latitudes.

In the chalk of Westphalia (also a marine deposit) a single specimen of another Astacomorph has been discovered, which possesses an especial interest as it is a true _Astacus_ (_A. politus_, Von der Marck and Schlüter), provided with the characteristic transversely divided telson which is found in the majority of the _Potamobiidæ_.

If we arrange the results of palæontological inquiry which have now been stated in the form of a table such as that which is given on the following page, the significance of the succession of astacomorphous forms, in time, becomes apparent. {345}

SUCCESSIVE FORMS OF THE ASTACOMORPHOUS TYPE.

I. Recent. _Potamobiidæ._ _Homarina._ _Penæus._ ────────────────────────────────────|─────────────────|────────────────────|── II. Later Tertiary. _Astacus_ | | | (Idaho). | | | ────────────────────────|───────────|─────────────────|────────────────────|── III. Earlier Tertiary. | | _Hoploparia._ | ────────────────────────|───────────|──────────────────────────────────────|── IV. Cretaceous. _Astacus._ _Pseudastacus._ _Enoploclytia._ _Hoploparia._ | ────────────────────────────\─────────────────────────────/────────────────|── V. Wealden \ / | (Fresh Water). \ / | ───────────────────────────────\───────────────────────/───────────────────|── VI. Jurassic. _Pseudastacus._ _Eryma._ _Penæus._ ─────────────────────────────────|────────────────────|────────────────────|── VII. Liassic. | _Eryma._ _Penæus._ ────────────────────────────────────────────────────────────────────────────── VIII. Triassic. ────────────────────────────────────────────────────────────────────────────── IX. Permian. ────────────────────────────────────────────────────────────────────────────── X. Carboniferous. _Anthrapalæmon._ ────────────────────────────────────────────────────────────────────────────── XI. Devonian. ────────────────────────────────────────────────────────────────────────────── XII. Silurian. ────────────────────────────────────────────────────────────────────────────── XIII. Cambrian.

If an Astacomorphous crustacean, having characters intermediate between those of _Eryma_ and those of _Pseudastacus_, existed in the Triassic epoch or earlier; if it gradually diverged into Pseudastacine and Erymoid forms; if these again took on Astacine and Homarine {346} characters, and finally ended in the existing _Potamobiidæ_ and _Homarina_, the fossil forms left in the track of this process of evolution would be very much what they actually are. Up to the end of the Mesozoic epoch the only known _Potamobiidæ_ are marine animals. And we have already seen that the facts of distribution suggest the hypothesis that they must have been so, at least up to this time.

Thus, with respect to the Ætiology of the crayfishes, all the known facts are in harmony with the requirements of the hypothesis that they have been gradually evolved in the course of the Mesozoic and subsequent epochs of the world’s history from a primitive Astacomorphous form.

And it is well to reflect that the only alternative supposition is, that these numerous successive and coexistent forms of insignificant animals, the differences of which require careful study for their discrimination, have been separately and independently fabricated, and put into the localities in which we find them. By whatever verbal fog the question at issue may be hidden, this is the real nature of the dilemma presented to us not only by the crayfish, but by every animal and by every plant; from man to the humblest animalcule; from the spreading beech and towering pine to the _Micrococci_ which lie at the limit of microscopic visibility.

{347}

NOTES.

NOTE I., CHAPTER I., p. 17.

THE CHEMICAL COMPOSITION OF THE EXOSKELETON.

The harder parts of the exoskeleton of the crayfish contain rather more than half their weight of calcareous salts. Of these nearly seven-eighths consist of carbonate of lime, the rest being phosphate of lime.

The animal matter consists for the most part of a peculiar substance termed _Chitin_, which enters into the composition of the hard parts not only of the _Arthropoda_ in general but of many other invertebrated animals. Chitin is not dissolved even by hot caustic alkalies, whence the use of solutions of caustic potash and soda in cleaning the skeletons of crayfishes. It is soluble in cold concentrated hydrochloric acid without change, and may be precipitated from its solution by the addition of water.

Chitin contains nitrogen, and according to the latest investigations (Ledderhose, “Ueber Chitin und seine Spaltungs-produkte:” Zeitschrift für Physiologische Chemie, II. 1879) its composition is represented by the formula C_{15}H_{26}N_{2}O_{10} .

NOTE II., CHAPTER I., p. 29.

THE CRAB’S EYES, OR GASTROLITHS.

The “Gastroliths,” as the “crab’s eyes” may be termed, are found fully developed only in the latter part of the summer season, just before ecdysis sets in. They then give rise to rounded prominences, one on {348} each side of the anterior part of the cardiac division of the stomach. The proper wall of the stomach is continued over the outer surface of the prominence; and, in fact, forms the outer wall of the chamber in which the gastrolith is contained, the inner wall being formed by the cuticular lining of the stomach. When the outer wall is cut through, it is readily detached from the convex outer surface of the gastrolith, with which it is in close contact. The inner surface of the gastrolith is usually flat or slightly concave. Sometimes it is strongly adherent to the chitonous cuticula; but when fully formed it is readily detached from the latter. Thus the proper wall of the stomach invests only the outer face of the gastrolith, the inner face of which is adherent to, or at any rate in close contact with, the cuticula. The gastrolith is by no means a mere concretion, but is a cuticular growth, having a definite structure. Its inner surface is smooth, but the outer surface is rough, from the projection of irregular ridges which form a kind of meshwork. A vertical section shows that it is composed of thin superimposed layers, of which the inner are parallel with the flat inner surface, while the outer becomes gradually concentric with the outer surface. Moreover, the inner layers are less calcified than the outer, the projections of the outer surface being particularly dense and hard. In fact, the gastroliths are very similar to other hard parts of the exoskeleton in structure, except that the densest layers are nearest the epithelial substratum, instead of furthest away from it.

When ecdysis occurs, the gastroliths are cast off along with the gastric armature in general, into the cavity of the stomach, and are there dissolved, a new cuticle being formed external to them from the proper wall of the stomach. The dissolved calcareous matter is probably used up in the formation of the new exoskeleton.

According to the observations of M. Chantran (Comptes Rendus, LXXVIII. 1874) the gastroliths begin to be formed about forty days before ecdysis takes place in crayfish of four years’ old; but the interval is less in younger crayfish, and is not more than ten days during the first year after birth. When shed into the stomach during ecdysis they are ground down, not merely dissolved. The process of destruction and absorption takes twenty-four to thirty hours in very young crayfish, seventy to eighty hours in adults. Unless the gastroliths are normally developed and re-absorbed, ecdysis is not healthily effected, and the crayfish dies in the course of the process. {349}

According to Dulk (“Chemische Untersuchung der Krebsteine:” Müller’s Archiv. 1835), the gastroliths have the following composition:—

Animal matter soluble in water 11·43 Animal matter insoluble in water (probably chitin) 4·33 Phosphate of lime 18·60 Carbonate of lime 63·16 Soda reckoned as carbonate 1·41 ───── 98·93 ─────

The proportion of mineral to animal matter and of phosphate to carbonate of lime is therefore greater in the gastroliths than in the exoskeleton in general.

NOTE III., CHAPTER I., p. 31.

GROWTH OF CRAYFISH.

The statements in the text, after the words “By the end of the year,” regarding the sizes of the crayfish at different ages, are given on the authority of M. Carbonnier (L’Écrevisse. Paris, 1869); but they obviously apply only to the large “Écrevisse à pieds rouges” of France, and not to the English crayfish, which appears to be identical with the “Écrevisse à pieds blancs,” and is of much smaller size. According to M. Carbonnier (l. c. p. 51), the young crayfish just born is “un centimètre et demi environ,” that is to say, three-fifths of an inch long. The young of the English crayfish still attached to the mother, which I have seen, rarely exceeds half this length.

M. Soubeiran (“Sur l’histoire naturelle et l’education des Écrevisses:” Comptes Rendus, LX. 1865) gives the result of his study of the growth of the crayfishes reared at Clairefontaine, near Rambouillet, in the following table:

Mean length. Mean weight. Metres. Grammes.

Crayfish of the year 0·025 0·50 Crayfish 1 year old 0·050 1·50 Crayfish 2 years old 0·070 3·50 Crayfish 3 years old 0·090 6·50 Crayfish 4 years old 0·110 17·50 Crayfish 5 years old 0·125 18·50 Crayfish indeterminate 0·160 30·00 Crayfish very old 0·190 125·00

These observations must also apply to the “Écrevisse à pieds rouges.”

{350}

NOTE IV., CHAPTER I., p. 37.

THE ECDYSES OF CRAYFISHES.

There is a good deal of discrepancy between different observers as to the frequency of the process of ecdysis in crayfishes. In the text I have followed M. Carbonnier, but M. Chantran (“Observations sur l’histoire naturelle des Écrevisses:” Comptes Rendus, LXXI. 1870, and LXXIII. 1871), who appears to have studied the question (on the “écrevisse à pieds rouges” apparently) very carefully, declares that the young crayfish moults no fewer than eight times in the course of the first twelve months. The first moult takes place ten days after it is hatched; the second, third, fourth, and fifth, at intervals of from twenty to twenty-five days, so that the young animal moults five times in the course of the ninety to one hundred days of July, August, and September. From the latter month to the end of April in the following year, no ecdysis takes place. The sixth takes place in May, the seventh in June, and the eighth in July. In the second year of its age, the crayfish moults five times, that is to say, in August and in September, and in May, June, and July following. In the third year, the crayfish commonly moults only twice, namely in July and in September. At a greater age than this, the females moult only once a year, from August to September; while the males moult twice, first in June and July; afterwards in August and September.

The details of the process of ecdysis are discussed by Braun, “Ueber die histologischen Vorgänge bei der Häutung von _Astacus fluviatilis_.” Würzburg Arbeiten, Bd. II.

NOTE V., CHAPTER I., p. 39.

REPRODUCTION IN CRAYFISHES.

The males are said to approach the females in November, December, and January, in the case of the French crayfishes. In England they certainly begin as early as the beginning of October, if not earlier. According to M. Chantran (Comptes Rendus, 1870), and M. Gerbe (Comptes Rendus, 1858), the male seizes the female with his pincers, throws her on her back, and deposits the spermatic matter, firstly, on the external plates of the caudal fin; secondly, on the thoracic sterna around the external openings of the oviducts. During this operation, the appendages of the two first abdominal somites are carried backwards, {351} the extremities of the posterior pair are inclosed in the groove of the anterior pair; and the end of the vas deferens becoming everted and prominent, the seminal matter is poured out, and runs slowly along the groove of the anterior appendage to its destination, where it hardens and assumes a vermicular aspect. The filaments of which it is composed are, in fact, tubular spermatophores, and consist of a tough case or sheath filled with seminal matter. The spoon-shaped extremity of the second abdominal appendage, working backwards and forwards in the groove of the anterior appendage, clears the seminal matter out of it, and prevents it from becoming choked.

After an interval which varies from ten to forty-five days, oviposition takes place. The female, resting on her back, bends the end of the abdomen forward over the hinder thoracic sterna, so that a chamber is formed into which the oviducts open. The eggs are passed into the chamber by one operation, usually during the night, and are plunged into a viscous greyish mucus with which it is filled. The spermatozoa pass out of the vermicular spermatophores, and mix with this fluid, in which the peculiarity of their form renders them readily recognisable. The spermatozoa are thus brought into close relation with the ova, but what actually becomes of them is unknown.

The origin of the viscous matter which fills the abdominal chamber when the eggs are deposited in it, and the manner in which these become fixed to the abdominal limbs is discussed by Lereboullet (“Recherches sur le mode de fixation des œufs aux faux pattes abdominaux dans les Écrevisses.” Annales des Sciences Naturelles, 4e Ee. T. XIV. 1860), and by Braun (Arbeiten aus dem Zoologisch-Zootomischen Institut in Würzburg, II.).

NOTE VI., CHAPTER I., p. 42.

ATTACHMENT OF THE YOUNG CRAYFISH TO THE MOTHER.

I observe that I had overlooked a passage in the Report on the award of the Prix Montyon for 1872, Comptes Rendus, LXXV. p. 1341, in which M. Chantran is stated to have ascertained that the young crayfishes fix themselves “en saisissant avec un de leurs pinces le filament qui suspend l’œuf à une fausse patte de la mère.”

In the paper already cited from the Comptes Rendus for 1870, M. Chantran states that the young remain attached to the mother during ten days after hatching, that is to say, up to the first moult. Detached before this period, they die; but after the first moult, they sometimes leave the {352} mother and return to her again, up to twenty-eight days, when they become independent.

In a note appended to M. Chantran’s paper, M. Robin states, that “the young are suspended to the abdomen of the mother by the intermediation of a chitinous hyaline filament, which extends from a point of the internal surface of the shell of the egg as far as the four most internal filaments of each of the lobes of the median membranous plate of the caudal appendage. The filaments exist when the embryos have not yet attained three-fourths of their development.” Is this a larval coat? Rathke does not mention it and I have seen nothing of it in those recently hatched young which I have had the opportunity of examining.

NOTE VII., CHAPTER II., p. 64.

THE “SALIVARY” GLANDS AND THE SO-CALLED “LIVER” OF THE CRAYFISH.

Braun (Arbeiten aus dem Zoologisch-Zootomischen Institut in Würzburg, Bd. II. and III.) has described “salivary” glands in the walls of the œsophagus, in the metastoma, and in the first pair of maxillæ of the crayfish.

Hoppe-Seyler (Pflügers Archiv, Bd. XIV. 1877) finds that the yellow fluid ordinarily found in the stomachs of crayfishes always contains peptone. It dissolves fibrin readily, without swelling it up, at ordinary temperatures; more quickly at 40° Centigrade. The action is delayed by even a trace of hydrochloric acid, and is stopped by the addition of a few drops of water containing 0.2 per cent. of that acid. By adding alcohol to the yellow fluid, a precipitate is obtained, which is soluble in water and in glycerine. The aqueous solution of the precipitate has a strong digestive action on fibrin, which is arrested by acidulation with hydrochloric acid. These reactions show that the fluid is very similar to, if not identical with, the pancreatic fluid of vertebrates.

The secretion of the “liver” taken directly from that gland, has a more strongly acid reaction than the fluid in the stomach, but has similar digestive properties. So has an aqueous extract of the gland, and a watery solution of the alcoholic precipitate. The aqueous extract also possesses a strong diastatic action on starch, and breaks up olive oil. There is no more glycogen in the “liver” than is to be found in other organs, and no constituents of true bile are to be met with.

{353}

NOTE VIII., CHAPTER II., p. 81.

ANAL RESPIRATION IN CRAYFISH.

Lereboullet (“Note sur une respiration anale observée chez plusieurs Crustacés;” Mémoires de la Société d’Histoire Naturelle de Strasbourg, IV. 1850) has drawn attention to what he terms “anal respiration” in young crayfish, in which he observed water to be alternately taken into and expelled from the rectum fifteen to seventeen times in a minute. I have never been able to observe anything of this kind in the uninjured adult animal, but if the thoracic ganglia are destroyed, a regular rhythmical dilatation and closing of the anal end of the rectum at once sets in, and goes on as long as the hindermost ganglia of the abdomen retain their integrity. I am much disposed to imagine that the rhythmical movement is inhibited, when the uninjured crayfish is held in such a position that the vent can be examined.

NOTE IX., CHAPTER II., p. 82.

THE GREEN GLAND.

The existence of guanin in the green gland rests on the authority of Will and Gorup-Besanez (Gelehrte Anzeigen, d. k. Baienzschen Akademie, No. 233, 1848), who say that in this organ and in the organ of Bojanus of the freshwater mussel, they found “a substance the reactions of which with the greatest probability indicate guanin,” but that they had been unable to obtain sufficient material to give decisive results.

Leydig (Lehrbuch der Histologie, p. 467) long ago stated that the green gland consists of a much convoluted tube containing granular cells disposed around a central cavity. Wassiliew (“Ueber die Niere des Flusskrebses:” Zoologischer Anzeiger, I. 1878) supports the same view, giving a full account of the minute structure of the organ, and comparing it with its homologues in the _Copepoda_ and _Phyllopoda_.

NOTE X., CHAPTER III., p. 105.

THE ANATOMY OF THE NERVOUS SYSTEM OF THE CRAYFISH.

The details respecting the origin and the distribution of the nerves are intentionally omitted. See the memoir by Lemoine of which the title is given in the “Bibliography.”

{354}

NOTE XI., CHAPTER III., p. 110.

THE FUNCTIONS OF THE NERVOUS SYSTEM OF THE CRAYFISH.

Mr. J. Ward, in his “Observations on the Physiology of the Nervous System of the Crayfish,” (Proceedings of the Royal Society, 1879) has given an account of a number of interesting and important experiments on this subject.

* * * * *

NOTE XII., CHAPTER III., p. 124.

THE THEORY OF MOSAIC VISION.

Oscar Schmidt (“Die Form der Krystalkegel im Arthropoden Auge:” Zeitschrift für Wissenschaftliche Zoologie, XXX. 1878) has pointed out certain difficulties in the way of the universal application of the theory of mosaic vision in its present form, which are well worthy of consideration. I do not think, however, that the substance of the theory is affected by Schmidt’s objections.

NOTE XIII., CHAPTER III., p. 135.

THE SPERMATOZOA.

Since the discovery of the spermatozoa of the crayfish in 1835–36 by Henle and von Siebold. the structure and development of these bodies have been repeatedly studied. The latest discussion of the subject is contained in a memoir of Dr. C. Grobben (“Beiträge zur Kenntniss der männlichen Geschlechtsorgane der Dekapoden:” Wien, 1878). There is no doubt that the spermatozoon consists of a flattened or hemispherical body, produced at its circumference into a greater or less number of long tapering curved processes (fig. 34 F). In the interior of this are two structures, one of which occupies the greater part of the body, and, when the latter lies flat, looks like a double ring. This may be called, for distinctness’ sake, the _annulate corpuscle_. The other is a much smaller _oval corpuscle_, which lies on one side of the first. The annulate corpuscle is dense, and strongly refracting; the oval corpuscle is soft, and less sharply defined. Dr. Grobben describes the annulate corpuscle as “napfartig,” or cup-shaped; closed below, open above, and with the upper edge turned inwards, and applied to the inner side of the wall of the cup. It appeared to me, on the other hand, that the annulate corpuscle is really a hollow ring, somewhat {355} like one of the ring-shaped air-cushions one sees, on a very small scale. Dr. Grobben describes the spermatoblastic cells of the testis and their nuclear spindles; but his account of the development of the spermatozoa does not agree with my own observations, which, so far as they have gone, lead me to infer that the annulate corpuscle of the spermatozoon is the metamorphosed nucleus of the cell from which the spermatozoon is developed. For want of material, however, I was unable to bring my investigations to a satisfactory termination, and I speak with reserve.

NOTE XIV., CHAPTER IV., p. 174.

THE MORPHOLOGY OF THE CRAYFISH.

The founder of the morphology of the _Crustacea_, M. Milne Edwards, counts the telson as a somite, and consequently considers that twenty-one somites enter into the composition of the body in the _Podophthalmia_. Moreover, he assigns the anterior seven somites to the head, the middle seven to the thorax, and the hinder seven to the abdomen. There is a tempting aspect of symmetry about this arrangement; but as to the limits of the head, the natural line of demarcation between it and the thorax seems to me to be so clearly indicated between the somite which bears the second maxillæ and that which carries the first maxillipedes in the _Crustacea_, and between the homologous somites in Insects, that I have no hesitation in retaining the grouping which I have for many years adopted. The exact nature of the telson needs to be elucidated, but I can find no ground for regarding it as the homologue of a single somite.

It will be observed that these differences of opinion turn upon questions of grouping and nomenclature. It would make no difference to the general argument if it were admitted that the whole body consists of twenty-one somites and the head of seven.

NOTE XV., CHAPTER IV., p. 199.

THE HISTOLOGY OF THE CRAYFISH.

In dealing with the histology of the crayfish I have been obliged to content myself with stating the facts as they appear to me. The discussion of the interpretations put upon these facts by other observers, especially in the case of those tissues, such as muscle, on which there is as yet no complete agreement even as to matters of observation, would require a whole treatise to itself.

{356}

NOTE XVI., CHAPTER IV., p. 221.

THE DEVELOPMENT OF THE CRAYFISH.

The remark made in the last note applies still more strongly to the history of the development of the crayfish. Notwithstanding the masterly memoir of Rathke, which constitutes the foundation of all our knowledge on this subject; the subsequent investigations of Lereboullet; and the still more recent careful and exhaustive works of Reichenbach and Bobretsky, a great many points require further investigation. In all its most important features I have reason to believe that the account of the process of development given in the text, is correct.

NOTE XVII., CHAPTER VI., p. 297.

PARASITES OF CRAYFISHES.

In France and Germany crayfishes (apparently, however, only _A. nobilis_) are infested by parasites, belonging to the genus _Branchiobdella_. These are minute, flattened, vermiform animals, somewhat like small leeches, from one-half to one-third of an inch in length, which attach themselves to the under side of the abdomen (_B. parasitica_), or to the gills (_B. astaci_), and live on the blood and on the eggs of the crayfish. A full account of this parasite, with reference to the literature of the subject, is given by Dormer (“Ueber die Gattung Branchiobdella:” Zeitschrift für Wiss. Zoologie, XV. 1865). According to Gay, a similar parasite is found on the Chilian crayfish. I have never met with it on the English crayfish. The Lobster has a somewhat similar parasite, _Histriobdella_. Girard, in the paper cited in the Bibliography, gives a curious account of the manner in which the little lamellibranchiate mollusk, _Cyclas fontinalis_, shuts the ends of the ambulatory limbs of crayfishes which inhabit the same waters, between its valves, so that the crayfish resembles a cat in walnut shells, and the pinched ends of the limbs become eroded and mutilated.

{357}

BIBLIOGRAPHY.

The subjoined list indicates the chief books and memoirs, in addition to those mentioned in the text and in the Appendix, which may be advantageously consulted by any one who wishes to study more fully the biology of the crayfishes.

I.—NATURAL HISTORY.

ROESEL VON ROSENHOF. Der Monatlich-herausgegeben Insekten Belustigung. 1755.

CARBONNIER. L’Écrevisse, Paris, 1869.

BRANDT AND RATZEBURG. Medizinische Zoologie. Bd. II., pp. 58–70.

BELL. British Stalk-eyed Crustacea, 1853.

SOUBEIRAN. Sur l’Histoire naturelle et l’Éducation des Écrevisses. Comptes Rendus, LX., 1865.

CHANTRAN. Observations sur l’Histoire naturelle des Écrevisses. Comptes Rendus, LXXI., 1870.

—— Sur la Fécondation des Écrevisses. Ibid., LXXIV., 1872.

—— Expériences sur la Régénération des Yeux chez les Écrevisses. Ibid., LXXVII., 1873.

—— Observations sur la Formation des Pierres chez les Écrevisses. Ibid., LXXVIII., 1874.

—— Sur le Mécanisme de la Dissolution intrastomacale des Concrétions gastriques des Écrevisses. Ibid., LXXVIII., 1874.

STEFFENBERG. Bijdrag til kanne domen on flodkraftens natural historia, 1872. Abstract in Zoological Record, IX.

VALLOT. Sur l’Écrevisse fluviatile et sur son parasite l’Astacobdelle branchiale. Comptes Rendus Acad. Sciences, Dijon. Mémoires, 1843–44. Dijon, 1845.

PUTNAM. On some of the Habits of the Blind Crayfish. Proceedings Boston Society of Nat. History, XVIII. {358}

HELLER. Ueber einen Flusskrebs-albino. Verhand d. Z. Bot. Gesellschaft, Wien. Bd. 7, 1857, and Bd. 8, 1858.

LEREBOULLET. Sur les variétés Rouge et Bleue de l’Écrevisse fluviatile. Comptes Rendus, XXXIII., 1857.

GIRARD. Quelques Remarques sur l’Astacus fluviatilis. Ann. Soc. Entom. France, T. VII. 1859.

II.—ANATOMY AND PHYSIOLOGY.

BRANDT AND RATZEBURG. _Op. cit._

MILNE EDWARDS. Histoire naturelle des Crustacés. 1834.

ROLLESTON. Forms of Animal Life. 1870.

HUXLEY. Manual of the Anatomy of Vertebrated Animals. 1877.

HUXLEY AND MARTIN. Elementary Biology. 1875.

SUCKOW. Anatomisch-Physiologische Untersuchungen. 1818.

KROHN. Verdauungsorgane des Krebses. Gefässsystem des Flusskrebses. Isis, 1834.

VON BAER. Ueber die sogenannte Erneuerung des Magens der Krebse und die Bedeutung der Krebssteine. Müller’s Archiv, 1835.

OESTERLEN. Ueber den Magen des Flusskrebses. Müller’s Archiv, 1840.

T. J. PARKER. On the Stomach of the Freshwater Crayfish. Journal of Anatomy and Physiology, 1876.

BARTSCH. Die Ernährungs- und Verdauungsorgane des _Astacus leptodactylus_. Budapester Naturhistor. Hefte II. 1878.

DESZŎ. Ueber das Herz des Flusskrebses und des Hummers. Zoologischer Anzeiger, I. 1878.

LEREBOULLET. Note sur une Respiration anale observée chez plusieurs Crustacées. Mém. de la Société d’Histoire Naturelle de Strasbourg, IV., 1850.

WASSILIEW. Ueber die Niere des Flusskrebses. Zoologischer Anzeiger, I. 1878.

LEMOINE. Recherches pour servir à l’histoire des systèmes nerveux, musculaire et glandulaire de l’Écrevisse. Annales des Sciences Naturelles, Sé. IV. T. 15, 1861.

DIETL. Die Organization des Arthropoden Gehirns. Zeitschrift für Wiss. Zoologie, XXVII., 1876.

KRIEGER. Ueber das centrale Nervensystem des Flusskrebses. Zoologischer Anzeiger, I., 1878.

LEYDIG. Das Auge der Gliederthiere. 1864. {359}

MAX SCHULZE. Die Zusammengesetzten Augen der Krebse und Insekten, 1868.

BERGER. Untersuchungen über den Bau des Gehirns und der Retina der Arthropoden. 1878.

GRENACHER. Untersuchungen über das Sehorgan der Arthropoden. 1879.

O. SCHMIDT. Die Form der Krystalkegel im Arthropoden Auge. Zeitschrift für Wiss. Zoologie, XXX., 1878.

FARRE. On the organ of hearing in the Crustacea. Phil. Trans. 1843.

LEYDIG. Ueber Geruchs- und Gehörorgane der Krebse und Insekten. Müller’s Archiv, 1860.

HENSEN. Studien über das Gehörorgan der Decapoden. Zeitschrift für Wissenschaftliche Zoologie, XIII. 1863.

GROBBEN. Beiträge zur Kenntniss der männlichen Geschlechtsorgane der Dekapoden. 1878.

BROCCHI. Recherches sur les Organes génitaux mâles des Crustacés décapodes. Annales des Sciences Naturelles, Sé. VI. ii.

LEYDIG. Zur feineren Bau der Arthropoden. Müller’s Archiv, 1855.

—— Handbuch der Histologie. 1857.

HAECKEL. Ueber die Gewebe des Flusskrebses. Müller’s Archiv, 1857.

BRAUN. Ueber die histologischen Vorgänge bei der Häutung von Astacus fluviatilis. Würzburg Arbeiten, II.

BAUR. Ueber den Bau der Chitinsehne am Kiefer des Flusskrebses und ihr Verhalten beim Schalenwechsel. Reichert u. Du Bois Archiv, 1860.

COSTE. Faits pour servir à l’Histoire de la Fécondation chez les Crustacés. Comptes Rendus, XLVI. 1858.

LEREBOULLET. Recherches sur la mode de Fixation des Œufs aux fausses pattes abdominales dans les Écrevisses. Annales des Sciences Naturelles, Sé. IV. T. 14, 1860.

III—DEVELOPMENT.

RATHKE. Ueber die Bildung und Entwickelung des Flusskrebses, 1829.

LEREBOULLET. Recherches d’Embryologie comparée sur le développement du Brochet, de la Perche et de l’Écrevisse. 1862. {360}

BOBRETSKY. (A Memoir in Russian, of which an abstract is given in Hofmann and Schwalbe, Jahresbericht für 1873 (1875)).

REICHENBACH. Die Embryonanlage und erste Entwickelung des Flusskrebses. Zeitschrift für Wiss. Zoologie. 1877.

IV.—TAXONOMY AND DISTRIBUTION OF CRAYFISHES.

A. _General._

MILNE EDWARDS. _Op. cit._

ERICHSON. Uebersicht der Arten der Gattung _Astacus_. Wiegmann’s Archiv für Naturgeschichte, XII. 1846.

DANA. Crustacea of the United States Exploring Expedition. 1852.

DE SAUSSURE. Note carcinologique sur la Famille des Thalassinides et sur celle des Astacides. Rev. et Magazin de Zoologie, IX.

HUXLEY. On the Classification and the Distribution of the Crayfishes. Proceedings of the Zoological Society. 1878.

B. _European and Asiatic._

RATHKE. Zur Fauna der Krym. 1836.

GERSTFELDT AND KESSLER. Cited in the text.

DE HAAN. Fauna Japonica. 1850.

LEREBOULLET. Description de deux nouvelles Espèces d’Écrevisses (_A. longicornis, A. pallipes_). Mém. Soc. Science Nat. Strasbourg. V. 1858.

HELLER. Crustaceen des südlichen Europa. 1863.

KESSLER. Ein neuer russischer Flusskrebs, _Astacus colchicus_. Bulletin de la Soc. Imp. des Naturalistes de Moscou, L. 1876.

C. _American._

STIMPSON. Crustacea and Echinodermata of the Pacific shores of North America. Journal of Boston Society of Natural History VI.; 1857–8.

DE SAUSSURE. Mémoire sur divers Crustacées nouveaux des Antilles et du Méxique. Mém. de la Société de Physique de Genève T. XIV., 1857.

VON MARTENS. Südbrasilische Süss- und Brackwasser Crustaceen (_A. pilimanus, A. brasiliensis_), Wiegmann’s Archiv, XXXV., 1869.

——. Ueber Cubansche Crustaceen. _Ibid._ XXXVIII.

HAGEN. Monograph of the North American _Astacidæ_. 1870. {361}

D. _Madagascar._

AUDOUIN AND MILNE EDWARDS. Sur une Espèce nouvelle du genre Écrevisse (_Astacus_). Écrevisse de Madagascar (_A. Madagascariensis_)., Mém. du Muséum d’Hist. naturelle, T. II. 1841.

E. _Australia._

VON MARTENS. On a new Species of _Astacus_. Annals & Mag. of Natural History, 1866.

HELLER. Reise der “Novara.” Zool. Theil. Bd. II. 1865.

F. _New Zealand._

MIERS. Notes on the Genera _Astacoides_ and _Paranephrops_. Transactions of the New Zealand Institute, IX., 1876.

—— _Paranephrops._ Zoology of “Erebus” and “Terror,” 1874. Catalogue of New Zealand Crustacea, 1876.

—— Annals of Natural History, 1876.

WOOD-MASON. On the mode in which the Young of the New Zealand _Astacidæ_ attach themselves to the Mother. Ann. & Mag. Natural History, 1876.

G. _Fossil Astacomorpha._

OPPEL. Palæontologische Mittheilungen, 1862.

BELL. British Fossil Crustacea. Palæontographical Society.

P. VAN BENEDEN. Sur la Découverte d’un Homard fossile dans l’Argile de Rupelmonde. Bulletin de l’Acad. Royale de Belgique. XXXIII., 1872.

VON DER MARCK UND SCHLÜTER. Neue Fische und Krebse von der Kreide von Westphalen. Palæontologica, XV. 1865.

COPE. On three extinct _Astaci_ from the freshwater tertiary of Idaho. Proceedings of the American Philosophical Society, XI., 1869–70.

{363}

INDEX.

A.

Abdomen, 19, 141 development of, 213

Abdominal appendages, 143 development of, 217

Abdominal somite, characters of, 142

Ætiology, 47

AGASSIZ, 308

Alimentary canal, 51 development of, 213, 222

Ambulatory legs, 168

American Crayfishes, 243, 247

_Amœba_, 285

Amurland Crayfishes, 304

Antenna, 23, 172 development of, 214, 218

Antennule, 23, 173 development of, 214, 218

_Anthrapalæmon_, 341

Anus, 29

Apodeme, 99, 158, 175

Appendage, 24, 143, 161, 173 abdominal, 143 cephalic, 170 thoracic, 164

Archenteron, 211

Arctogæal province, 314

Areola, 235

ARISTOTLE, referred to, 4

Arteries, 71

Arteries, development of, 224

Arthrobranchia, 75

Arthrophragm, 158

_Arthropoda_, 279, 284

Articulations, 95

Asiatic Crayfishes, 304

_Astacina_, 254

_Astacoides_, 250, 313

_Astacomorpha_, 338

_Astacopsis_, 250, 264

_Astacus_, division into sub-genera, 290

_Astacus angulosus_, 302, 310 _colchicus_, 302, 310 _dauricus_, 304, 310 _fluviatilis_, anatomy, general account of, 17–31 attachment of young to mother, 40, 351 branchial formula, 266 development, 205–226 distribution, geographical 44, 288, 298 distribution, chronological, 44 ecdysis, 32, 350 general characters, 6 growth, 31, 349 habits, 8 {364} histology, 174 mortality, 127 muscular system, 90 myths concerning, 44 name, origin of, 13 nervous system, 101 newly hatched young, characters of, 219 nutrition, 48 occurrence, 5, 8 organs of alimentation, 51 circulation, 68 excretion, 82, 353 hearing, 116 reproduction, 128 respiration, 75, 353 sight, 118 smell, 114 taste, 115 touch, 113 prehension of food, 49 putrid, effect of smell of, 45 reproduction of lost limbs, 38 reproduction, sexual, 39, 128, 135, 350 sexual characters, 7, 20, 32, 145, 241 somites and appendages, 143 systematic description, 230 use as food, 10, 289 varieties, 289 _fontinalis_, 290 _japonicus_, 304 _klamathensis_, 305 _leniusculus_, 305 _leptodactylus_, 299, 302, 303, 310, 320 _nigrescens_, 244 _nobilis_, 290, 295, 296, 299, 310 _oreganus_, 305 _pachypus_, 302, 310 _pallipes_, 290 _politus_, 344 _saxatilis_, 290 _Schrenckii_, 304, 310 _torrentium_, 290, 294, 298, 310, 311 _tristis_, 290 _Trowbridgii_, 305

_Atya_, _Atyidæ_, 331, 336

Auditory organ, 116 setæ, 116

Australian Crayfishes, 306 province, 314

Austrocolumbian province, 314

_Axius_, 271

B.

Ball, R., quoted, 36

Basipodite, 143

BELL, T., quoted, 37, 42

Bile-duct, 61, 66

Biological sciences, scope of, 4

Blastoderm, 207

Blastomere, 205

Blastopore, 209

Blood, 31, 68, 176 corpuscles, 69, 176 development of, 224 sinuses, 50, 69

BOBRETSKY, referred to, 356

BOLIVAR, Dr., 298

Branchiæ, _Astacoides_, 266 _Astacopsis_, 264 {365} _Astacus_, 25, 75, 265 development of, 224 _Cancer_, 276 _Homarus_, 257 _Palæmon_, 270 _Palinurus_, 264 _Penæus_, 267

Branchial chamber, 25 formula, _Astacoides_, 266 _Astacopsis_, 264 _Astacus_, 266 _Cancer_, 277 hypothetically complete, 268 _Palæmon_, 270 _Palinurus_, 265 _Penæus_, 267

_Branchiobdella_, 356

Branchiostegite, 25 development of, 217

BRAUN, quoted, 352

Brazilian Crayfishes, 306

C.

Cæcum, 61

Calcification of exoskeleton, 197

Californian Crayfishes, 243

_Cambarus_, 44, 247, 310, 312

_Cancer_, 272, 283

Carapace, 19 development of, 214

CARBONNIER, M., quoted, 297, 349, 350

Cardia, 52

_Caridina_, 330

Carpopodite, 165

Cell, 66, 199

Cell-aggregate, 190, 199 division, 200 theory, 202, 204

Cephalic appendages, 170 development of, 217 flexure, 163 somites, 154

Cephalon, 19, 141

Cephalothorax, 19

Cervical groove, 19 spines, 234

CHANTRAN, M., quoted, 348, 350, 351

Chelæ, 22

Chilian Crayfishes, 308

Chitin, 50 composition of, 347

_Chæraps_, 250

Chorology, 46

Circulation, 73 organs of, 68

Common knowledge and science, 3

Connective tissue, 178 development of, 224

COPE, Prof., quoted, 316

Cornea, 118

Coxopodite, 143

Coxopoditic setæ, 78

Crab, see _Cancer_

Crab’s-eye, see Gastrolith

_Crangon_, 272

Crayfish, origin of name, 12 common, see _Astacus fluviatilis_

Crayfishes, Amurland, 304 Asiatic, 304 Australian, 306 Brazilian, 306 Californian, 243 Chilian, 308 definition of, 254 Eastern North American, 247, 305 European, 288, 297 evolution of, 331 Figian, 306, 313 Japanese, 304, 313 Mascarene, 308, 313 northern and southern, compared, 252 Novozelanian, 306, 313 southern, 249 Tasmanian, 306 Western North American, 305, 313

_Crustacea_, 271, 278

Crystalline cones, 121

Cuticle, 33, 50, 175, 192

_Cyclas_, 356

D.

Dactylopodite, 165

_Daphnia_, asexual reproduction of, 128

DARWIN, C., referred to, 4

DE HAAN, quoted, 313

Development, 205 abdomen, 213 abdominal appendages, 217 alimentary canal, 213, 222 antennæ, 214, 218 antennules, 214, 218 blood and blood vessels, 224 branchiostegite, 217 carapace, 214 cephalic appendages, 217, 219 connective tissue, 224 ear, 225 eye, 225 eyestalk, 214, 218 gills, 224 heart, 224 kidney, 224 labrum, 218 mandibles, 214 muscles, 224 nervous system, 213, 224 reproductive organs, 225 rostrum, 217 thoracic appendages, 217, 219

Digestion, 63

Distribution, 46 chronological, of crayfishes, 44, 316, 339 table of, 345 geographical, of crayfishes, 44, 288 causes of, 335 results of study of, 308, 314

DORMER, quoted, 356

DULK, quoted, 349

E.

Ear, 116 development of, 225

Ecdysis, 32, 350

Écrevisse à pieds blancs, 289, 297 à pieds rouges, 289, 297

Ectoderm, 141

Ectostracum, 194

Edelkrebs, 290

Endoderm, 141

Endophragmal system, 157

Endopleurite, 158

Endopodite, 145

Endoskeleton, 17

Endosternite, 158

Endostracum, 194

_Engæus_, 250, 306

_Enoplocytia_, 342

Epiblast, 211

Epidermis, 140

Epimeron, 143

Epiostracum, 192

Epipodite, 167 {367}

Epistoma, 155

Epithelium, 140, 177

_Equus excelsus_, occurring with fossil crayfishes, 316

_Eryma_, 341

Evolution of crayfishes, 331

Excretion, organs of, 82

Exopodite, 145

Exoskeleton, 17 chemical composition, 347

Eye, 118 compound, 122 development of, 225

Eye-stalk, 24, 173 development of, 214

F.

Family, 252

Fat-cells, 180

Fibre, muscular, 185

Fibril, muscular, 185

Figian Crayfishes, 306

Filament, muscular, 185

Filter of stomach, 58

Flagellum, 167

Food-yelk, 206

Foot-jaws, _see_ maxillipedes

Forceps, 22

Foregut, 61 development of, 213, 222

Fossil crayfishes, 316

FOSTER, Dr. M., referred to, 110

France, consumption of crayfish in, 10

Function, 22

G.

_Galaxidæ_, 315

_Gammarus_, 323

Ganglion, 103, 105

Ganglionic corpuscle, 87, 103

Gastric mill, 53

Gastrolith, 29, 347 chemical composition, 349

Gastrula, 211

GAY, quoted, 356

Genus, 249

Geographical distribution, see Distribution

GERBE, M., quoted, 350

Germinal disc, 209 layer, 206 spot, 133 vesicle, 133

GERSTFELDT, Dr., quoted, 290

Gills, see Branchiæ

GIRARD, quoted, 356

GORUP-BESANEZ, quoted, 353

Green-gland, 83, 353 development of, 224

GROBBEN, Dr., quoted, 354

Growth of crayfish, 31, 349

Guanin, 82, 353

Gullet, see Œsophagus

GÜNTHER, Dr., quoted, 315

H.

HAGEN, Dr., quoted, 305, 312

_Haplochitonidæ_, 315

HARVEY, quoted, 5

Head, see Cephalon

Hearing, organ of, 116

Heart, 27, 71 development of, 224

HELLER, Dr., quoted, 298, 330

Hepatic duct, see Bile duct

Hind gut, 61 development of, 214, 223

Histology, 176

_Histriobdella_, 356

_Homaridæ_, 263 {368}

_Homarina_, 261

_Homarus_, 13, 42, 257, 332

Homology, homologous, homologue, 148

_Hoploparia_, 342

Hypoblast, 211

I.

_Idothea_, 323, 334

Impregnation, 135, 350

Integument, 50

Interseptal zone, 183

Intestine, 29, 61

Ischiopodite, 165

J.

Japanese Crayfishes, 313, 314

Jaws, 23

JOHNSTON, J., quoted, 42

K.

KESSLER, quoted, 298, 304

Kidney, see Green gland

KLUNZINGER, referred to, 330

L.

Labrum, 51 development of, 218

LAMARCK, referred to, 4

LEREBOULLET, quoted, 353

Legs, ambulatory, 168

LEMOINE, referred to, 353

LEYDIG, referred to, 115, 353

Liver, 30, 64 development of, 223 nature of secretion, 352

Lobster, common, see _Homarus_ Norway, see _Nephrops_ Rock, see _Palinurus_

LOVÈN, referred to, 327

M.

Machine, living, 128

M^CINTOSH, Dr. W. C., quoted, 288

Mandible, 23, 51, 170 development of, 214

MARTENS, VON, 306

_Mastodon mirificus_, occurring with fossil crayfishes, 316

Maxillæ, 23, 170

Maxillipedes, 23, 164

Medullary groove, 213

Megalopa stage of development, 283

Meropodite, 165

Mesoblast, 212

Mesoderm, 141

Mesophragm, 158

Metamere, 143

Metastoma, 51

Metope, 278

Midgut, 61 development of, 211, 214, 223

MILNE-EDWARDS, quoted, 13, 289

_Mollusca_, 284

Morphology, 46, 138 comparative, 230

Mortality of crayfishes, 128

Morula, 206

Mosaic vision, 122, 354

Motor plates, 189

Mouth, 51

MÜLLER, JOHANNES, referred to, 122

Muscle, 57, 90, 175, 181 development of, 224 histology of, 90, 181

Muscles of abdomen, 99 {369} of chela, 93 of stomach, 57

Myosin, 186

Myotome, 174

_Mysis_, 281, 323 _relicta_, origin of, from _M. oculata_, 327

Mysis stage of development, 280

N.

Natural History, 3 Philosophy, 3

Nauplius stage of development, 215, 280

Nearctic province, 314

_Nephrops_, 259, 332

Nerve, 101 auditory, 117 optic, 118

Nerve-cells, 103, 187 fibres, 101, 188

Nervous system, 105 development of, 213, 224 functions of, 354

Noble crayfish, see _Astacus nobilis_

Nomenclature, binomial, 13, 15

Norway lobster, see _Nephrops_

Novozelanian province, 314

Nucleated cell, 199

Nucleolus, 187

Nucleus, 177, 200 changes of, in cell-division, 200

O.

Œsophagus, 51

Olfactory organ, 114

Organ, 22

Origin of crayfish, evidence as to, 320, 331

Ovary, 31, 129 structure of, 131

Oviduct, 129

Oviposition, 351

Ovisac, 132

Ovum, 129 structure of, 133

P.

Palæarctic province, 314

_Palæmon_, 268, 328

_Palinuridæ_, 263

_Palinurus_, 261, 264

Palp, 171

_Paranephrops_, 250, 306, 313

Paraphragm, 158

Parasites of crayfish, 356

_Parastacidæ_, 252, 256, 306, 313

_Parastacus_, 250, 306

_Pemphix_, 341

_Penæus_, 267, 280

Pericardium, 69

Perivisceral cavity, 50

Phyllobranchia, 271

Physiology, 46

Pleurobranchia, 79

Pleuron, 96, 143

Podobranchia, 75, 165

_Podophthalmia_, 279

Pore-canals, 195

Post-orbital ridge, 233 spine, 232

_Potamobiidæ_, 252, 256

Prawn, see _Palæmon_

Prehension of food, 49

Procephalic lobes, 160 development of, 213

Propodite, 165

Protopodite, 143

_Prototroctes_, 315 {370}

_Protozoa_, 285

_Pseudastacus_, 343

Pylorus, 52

R.

Race, 292

RATHKE, quoted, 356

RÉAUMUR, quoted, 33

Reflex action, 108

REICHENBACH, quoted, 356

Renal organ, see Green-gland

Reproduction of lost limbs, 38 sexual, 39, 128, 135, 350

Reproductive organs, 128 development of, 225

Respiration, anal, 353

Respiratory organs, see Branchiæ

_Retropinna_, 315

ROBIN, quoted, 352

Rock lobster, see _Palinurus_

ROESEL VON ROSENHOF, quoted, 41, 43

RONDOLETIUS, referred to, 4

Rostrum, 157 development of, 217

S.

Salivary glands, 352

_Salmonidæ_, parallel between their distribution, and that of _Astacidæ_, 315

Sarcolemma, 90, 182

SARS, G. O., referred to, 327

SARTORIUS VON WALTERHAUSEN, quoted, 322

Scaphognathite, 80, 170

Schizopod stage of development, 280

SCHLÜTER, 317

SCHMIDT, O., quoted, 354

SCHRANK, 290

Science, physical, 3

Science and common sense, 1

Segmentation, 174

Self-causation, 112

Sensory organs, 113

Septal line, 183 zone, 183

Setæ, 197

Shrimp, see _Crangon_

SIEBOLD, VON, referred to, 331

Sight, organ of, 118

Sinus, sternal, 69

Smell, organ of, 114

Somite, 143, 161, 355 abdominal, 142 cephalic, 154 thoracic, 150

SOUBEIRAN, M., quoted, 349

Southern Crayfishes, 249

Species, 243, 290 morphological, 291 physiological, 296

Spermatozoa, 129, 135, 354

Spontaneous action, 112

Squame of antenna, 172

Steinkrebs, see _Astacus torrentium_

Sternum, 96, 143

Stomach, 29, 51

Stone-crayfish, see _Astacus torrentium_

Striated spindle, 121

Swimmeret, 20

T.

Taste, organ of, 115

Teleology, 47, 137

Tendon, 92, 175 {371}

Tergum, 96, 143

Terminal plates, 189

Terminology, scientific, 14

Testis, 129 structure of, 133

Thoracic appendages, 164 development of, 217 somites, 150

Thorax, 19, 141

Tissue, 175

Touch, organ of, 113

Transformism, 318

TREVIRANUS, referred to, 4

Tribe, 252

Trichobranchiæ, 263

_Troglocaris_, 337

V.

Valves of heart, 73 of stomach, 59

VAN HELMONT, quoted, 45

Variety, 290, 292

Vas deferens, 130

Vent, see Anus

_Vertebrata_, 284 eye of, 122, 125

Visual pyramid, 121 rod, 121

Vitelline membrane, 133

Vitellus, 133

Voluntary action, 112

VON DER MARCK, 317

W.

WARD, J., referred to, 354

WASSILIEW, quoted, 353

Whirlpool of life, 84

WILL, quoted, 353

WOOD-MASON, quoted, 44

Y.

Yelk, 133

Yelk-division, 205

Young of _Astacus_, newly hatched, characters of, 219

Z.

Zoæa stage of development, 280

PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES.

* * * * * *

Transcriber’s note:

Original spelling and grammar have been generally retained, with some exceptions noted below.

Ditto marks have been removed. In the _List of Woodcuts_, the ditto marks have been replaced with em dashes.

Footnotes have been relabeled 1–43 and moved from within paragraphs to nearby locations between paragraphs.

Illustrations have been moved from within paragraphs to nearby locations between paragraphs. Therefore, the page numbers shown in the List of Woodcuts are sometimes wrong.

On page 176 and elsewhere there are phrases such as ‹rarely exceed 1‐700th›, in which the fraction appears to have been printed with a hyphen. In other places, e.g. page 19, there are fractions such as ‹1/110th›, in which the ‹1› was actually printed over a vinculum, over ‹110›, in small text, and this fraction was followed by ‹th› in regular sized text. Herein, both forms have been converted to a form like this: ‹1‐30,000th› (e.g. from page 182), using the Unicode point [u+2010 hyphen].

Page xi. The List of Woodcuts was restructured slightly to form a simple list. Ditto marks and curly brackets were removed.

Page 74. The phrase ‹that it to say› was changed to ‹that is to say›.

Page 102. The phrase ‹fibre fig. 19, F.)› was changed to ‹fibre (fig. 19, F).›.

Pages 228, 292. The word ‹develope› was changed to ‹develop›.

Page 282, Fig. 74. Left parenthesis was inserted before ‹The figures A›.

Pages 288, 368. The name ‹M’Intosh› appears on page 288, but this name is spelled in the Index on page 368 with what would now be represented by the Unicode character [ʻ U+02BB; MODIFIER LETTER TURNED COMMA] in place of the right single quotation mark. U+02BB is probably correct, but several current browsers don’t support this character. So both instances are spelled herein ‹M^cIntosh›.

Page 298. The word ‹kindess› was changed to ‹kindness›.

Page 314. The word ‹oocupied› was changed to ‹occupied›.

Page 345. The graphic chart, _Successive Forms of the Astacomorphous Type_, is poorly represented in our text edition, due to the inherent limitations of plain text. An image is used in the html edition.

Page 363. The original INDEX was structured loosely as a nested list. In the simple text edition, this structure has been reproduced as well as possible. In the html/epub/mobi editions, an html nested list structure has been imposed. Html list code is more rigidly semantic than the original printed index, so some corrections had to be applied, and ambiguities were necessarily clarified. A few mistakes are possible during such a process.