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.
[Illustration: FIG. 75.—Astacus leptodactylus (after Rathke, 1/3 nat.
size).]
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.
[Illustration: FIG. 76.—Australian Crayfish (1/3 nat. size).[33]]
[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.
[Illustration: FIG. 77.—MAP OF THE WORLD, showing the geographical
distribution of the Crayfishes. I. Eur-asiatic Crayfishes; II.
Amurland Crayfishes; III. Japanese Crayfishes; IV. Western North
American Crayfishes; V. Eastern North American Crayfishes; VI.
Brazilian Crayfishes; VII. Chilian Crayfishes; VIII. Novozelanian
Crayfishes; IX. Fijian Crayfishes; X. Tasmanian Crayfishes; XI.
Australian Crayfishes; XII. Mascarene 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.
[Illustration: FIG. 78.—_Cambarus_ (Guatemala) penultimate leg.
_cxp_, coxopodite; _cxs_, coxopoditic setæ; _pdb_, podobranchia;
_bp_, basipodite; _ip_, ischiopodite; _mp_, meropodite; _cp_,
carpopodite; _pp_, propodite; _dp_, dactylopodite.]
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_.
[Illustration: FIG. 79. _Palæmon jamaicensis_ (about 5/7 nat. size).
A, female; B, fifth thoracic appendage of male.]
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.
[Illustration: FIG. 80.—A, _Pseudastacus pustulosus_ (nat. size). B,
_Eryma modestiformis_ (× 2). Both figures after Oppel.]
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.
[Illustration: FIG. 81.—_Hoploparia longimana_ (2/3 nat. size).—_cp_,
carapace; _r_, rostrum, T, telson; XV., XVI., first and second
abdominal somites; 10, forceps; 20, last abdominal appendage.]
_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.