THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.—THE STRUCTURE AND THE
DEVELOPMENT OF THE CRAYFISH COMPARED WITH THOSE OF OTHER LIVING
BEINGS.
Up to this point, our attention has been directed almost exclusively
to the common English crayfish. Except in so far as the crayfish is
dependent for its maintenance upon other animals, or upon plants,
we might have ignored the existence of all living things except
crayfishes. But, it is hardly necessary to observe, that innumerable
hosts of other forms of life not only tenant the waters and the
dry land, but throng the air; and that all the crayfishes in the
world constitute a hardly appreciable fraction of its total living
population.
Common observation leads us to see that these multitudinous living
beings differ from not-living things in many ways; and when the
analysis of these differences is pushed as far as we are at present
able to carry it, it shews us that all living beings agree with the
crayfish and differ from not-living things in the same particulars.
Like the crayfish, they are constantly wasting away by {228}
oxidation, and repairing themselves by taking into their substance
the matters which serve them for food; like the crayfish, they shape
themselves according to a definite pattern of external form and
internal structure; like the crayfish, they give off germs which grow
and develop into the shapes characteristic of the adult. No mineral
matter is maintained in this fashion; nor grows in the same way; nor
undergoes this kind of development; nor multiplies its kind by any
such process of reproduction.
Again, common observation early leads to the discrimination of
living things into two great divisions. Nobody confounds ordinary
animals with ordinary plants, nor doubts that the crayfish belongs
to the former category and the waterweed to the latter. If a living
thing moves and possesses a digestive receptacle, it is held to be
an animal; if it is motionless and draws its nourishment directly
from the substances which are in contact with its outer surface,
it is held to be a plant. We need not inquire, at present, how far
this rough definition of the differences which separate animals
from plants holds good. Accepting it for the moment, it is obvious
that the crayfish is unquestionably an animal,—as much an animal
as the vole, the perch, and the pond-snail, which inhabit the same
waters. Moreover, the crayfish has, in common with these animals, not
merely the motor and digestive powers characteristic of animality,
but they all, like it, possess a complete alimentary canal; special
{229} apparatus for the circulation and the aëration of the blood;
a nervous system with sense-organs; muscles and motor mechanisms;
reproductive organs. Regarded as pieces of physiological apparatus,
there is a striking similarity between all three. But, as has already
been hinted in the preceding chapter, if we look at them from a
purely morphological point of view, the differences between the
crayfish, the perch, and the pond-snail, appear at first sight so
great, that it may be difficult to imagine that the plan of structure
of the first can have any relation to that of either of the last two.
On the other hand, if the crayfish is compared with the water-beetle,
notwithstanding wide differences, many points of similarity between
the two will manifest themselves; while, if a small lobster is set
side by side with a crayfish, an unpractised observer, though he will
readily see that the two animals are somewhat different, may be a
long time in making out the exact nature of the differences.
Thus there are degrees of likeness and unlikeness among animals, in
respect of their outward form and internal structure, or, in other
words, in their morphology. The lobster is very like a crayfish,
the beetle is remotely like one; the pond-snail and the perch
are extremely unlike crayfishes. Facts of this kind are commonly
expressed in the language of zoologists, by saying that the lobster
and the crayfish are closely allied forms; that the beetle and the
crayfish present a remote affinity; and that there is no affinity
between the {230} crayfish and the pond-snail, or the crayfish and
the perch.
The exact determination of the resemblances and differences of animal
forms by the comparison of the structure and the development of one
with those of another, is the business of comparative morphology.
Morphological comparison, fully and thoroughly worked out, furnishes
us with the means of estimating the position of any one animal in
relation to all the rest; while it shews us with what forms that
animal is nearly, and with what it is remotely, allied: applied to
all animals, it furnishes us with a kind of map, upon which animals
are arranged in the order of their respective affinities; or a
classification, in which they are grouped in that order. For the
purpose of developing the results of comparative morphology in the
case of the crayfish, it will be convenient to bring together, in a
summary form, those points of form and structure, many of which have
already been referred to and which characterise it as a separate kind
of animal.
* * * * *
Full-grown English crayfishes usually measure about three inches and
a half from the extremity of the rostrum in front to that of the
telson behind. The largest specimen I have met with measured four
inches.[13] The {231} males are commonly somewhat larger, and they
almost always have longer and stronger forceps than the females. The
general colour of the integument varies from a light reddish-brown
to a dark olive-green; and the hue of the tergal surface of the body
and limbs is always deeper than that of the sternal surface, which is
often light yellowish-green, with more or less red at the extremities
of the forceps. The greenish hue of the sternal surface occasionally
passes into yellow in the thorax and into blue in the abdomen.
[13] The dimensions of crayfishes at successive ages given
at p. 31, beginning at the words “By the end of the year,”
refer to the “écrevisse à pieds rouges” of France; not to the
English crayfish, which is considerably smaller. Doubtless, the
proportional rate of increment is much the same, in the two
kinds; but in the English crayfish it has not been actually
ascertained.
The distance from the orbit to the posterior margin of the carapace
is nearly equal to that from the posterior margin of the carapace
to the base of the telson, when the abdomen is fully extended, but
this measurement of the carapace is commonly greater than that of the
abdomen in the males and less in the females.
The general contour of the carapace (fig. 61), without the rostrum,
is that of an oval, truncated at the ends: the anterior end being
narrower than the posterior. Its surface is evenly arched from side
to side. The greatest breadth of the carapace lies midway between the
cervical groove and its posterior edge. Its greatest vertical depth
is on a level with the transverse portion of the cervical groove.
The length of the rostrum, measured from the orbit {232} to its
extremity, is greater than half the distance from the orbit to the
cervical groove. It is trihedral in section, and its free end is
slightly curved upwards (fig. 41). It gradually becomes narrower for
about three-fourths of its whole length. At this point it has rather
less than half the width which it has at its base (fig. 61, A); and
its raised, granular and sometimes distinctly serrated margins are
produced into two obliquely directed spines, one on each side. Beyond
these, the rostrum rapidly narrows to a fine point; and this part of
the rostrum is equal in length to the width between the two spines.
The tergal surface of the rostrum is flattened and slightly excavated
from side to side, except in its anterior half, where it presents a
granular or finely serrated median ridge, which gradually passes into
a low elevation in the posterior half, and, as such, may generally be
traced on to the cephalic region of the carapace. The inclined sides
of the rostrum meet ventrally in a sharp edge, convex from before
backwards; the posterior half of this edge gives rise to a small,
usually bifurcated, spine, which descends between the eye-stalks
(fig. 41). The raised and granulated lateral margins of the rostrum
are continued back on to the carapace for a short distance, as two
linear ridges (fig. 61, A). Parallel with each of these ridges, and
close to it, there is another longitudinal elevation (_a, b_), the
anterior end of which is raised into a prominent spine (_a_), which
is situated immediately behind the orbit, and may, therefore, be
termed the _post-orbital_ {233} _spine_. The elevation itself may
be distinguished as the _post-orbital ridge_. The flattened surface
of this ridge is marked by a longitudinal depression or groove. The
{234} posterior end of the ridge passes into a somewhat broader and
less marked elevation, the hinder end of which turns inwards, and
then comes to an end at a point midway between the orbit and the
cervical groove. Generally this hinder elevation appears like a mere
continuation of the post-orbital ridge; but, sometimes, the two are
separated by a distinct depression. I have never seen any prominent
spine upon the posterior elevation, though it is sometimes minutely
spinulose. The post-orbital ridges of each side, viewed together,
give rise to a characteristic lyrate mark upon the cephalic region of
the carapace.
[Illustration: FIG. 61.—A, D, & G, _Astacus torrentium_; B, E, & H,
_A. nobilis_; C, F, & I, _A. nigrescens_ (nat. size). A–C, Dorsal
views of carapace; D–F, side views of third abdominal somites; G–I,
Dorsal views of telson. _a, b_, post-orbital ridge and spines; _c_,
branchio-cardiac grooves inclosing the areola.]
A faintly marked, curved, linear depression runs from the hinder end
of the post-orbital ridge, at first directly downwards, and then
curves backwards to the cervical groove. It corresponds with the
anterior and inferior boundary of the attachment of the adductor
muscle of the mandible.
Below the level of this, and immediately behind the cervical groove,
there are usually three spines, arranged in a series, which follow
the cervical groove. The points of all are directed obliquely
forwards, and the lowest is the largest. Sometimes there is only one
prominent spine, with one or two very small ones; sometimes there are
as many as five of these _cervical spines_.
The cardiac region is marked out by two grooves which run backwards
from the cervical groove (fig. 61, A, _c_), and terminate at a
considerable distance from the posterior {235} edge of the carapace.
Each groove runs, at first, obliquely inwards, and then takes a
straight course parallel with its fellow. The area thus defined is
termed the _areola_; its breadth is equal to about one-third of the
total transverse diameter of the carapace in this region.
No such distinct lines indicate the lateral boundary of the region
in front of the cervical groove which answers to the stomach. But
the middle part of the carapace, or that which is comprised in
the gastric and cardiac regions, has its surface sculptured in a
different way from the branchiostegites and the lateral regions of
the head. In the former, the surface is excavated by shallow pits,
separated by relatively broad flat-topped ridges; but, in the latter,
the ridges become more prominent, and take the form of tubercles, the
apices of which are directed forwards. Minute setæ spring from the
depressions between these tubercles.
The branchiostegite has a thickened rim, which is strongest below and
behind (fig. 1). The free edge of this rim is fringed with close-set
setæ.
The pleura of the second to the sixth abdominal somites are broadly
lanceolate and obtusely pointed at their free ends (fig. 61, D); the
anterior edge is longer and more convex than the posterior edge. In
the females, the pleura are larger, and are directed more outwards
and less downwards than in the males. The pleura of the second somite
are much larger than the rest, and overlap the very small pleura of
the first somite (fig. 1). The {236} pleura of the sixth somite are
narrow, and their posterior edges are concave.
The pits and setæ of the cuticle which clothes the tergal surfaces
of the abdominal somites are so few and scattered, that the latter
appear almost smooth. In the telson, however, especially in its
posterior division, the markings are coarser and the setæ more
apparent.
The telson (fig. 61, G) presents an anterior quadrate division and
a posterior half-oval part, the free curved edge of which is beset
with long setæ, and is sometimes slightly notched in the middle.
The posterior division is freely movable upon the anterior, in
consequence of the thinness and pliability of the cuticle along
a transverse line which joins the postero-external angles of the
anterior division, each of which is produced into two strong spines,
of which the outer is the longer. The length of the posterior
division of the telson, measured from the middle of the suture, is
equal to, or but very little less than, that of the anterior division.
On the under side of the head, the basal joints of the antennules are
visible, internal to those of the antennæ, but the attachment of the
latter is behind and below that of the former (fig. 3, A). Behind
these, and in front of the mouth, the epistoma (fig. 39, A, II, III)
presents a broad area of a pentagonal form. The posterior boundary of
this area is formed by two thickened transverse ridges, which meet
on the middle line at a very open angle, the apex of which is turned
forwards. {237} The posterior edges of these ridges are continuous
with the labrum. The anterior margin is produced in the middle into a
_fleur de lys_ shaped process, the summit of which terminates between
the antennules. At the sides of this process, the anterior margin
of the epistoma is deeply excavated to receive the basal joints of
the antennæ. Following the contours of these excavated margins, the
surface of the epistoma presents two lateral convexities. The widest
and most prominent part of each of these lies towards the outer edge
of the epistoma, and is produced into a conical spine. Sometimes
there is a second smaller spine beside the principal one. Between the
two convexities lies a triangular median depressed area.
The distance from the apex of the anterior median process to the
posterior ridge is equal to a little more than half the width of the
epistoma.
The corneal surface of the eye is transversely elongated and
reniform, and its pigment is black. The eye-stalks are much broader
at their bases than at their corneal ends (fig. 48, A). The
antennules are about twice as long as the rostrum. The tergal surface
of the trihedral basal joint of the antennule, on which the eye-stalk
rests, is concave; the outer surface is convex, the inner flat
(figs. 26, A, and 48, B). Near the anterior end of the sternal edge
which separates the two latter faces, there is a strong curved spine
directed forwards (fig. 48, B, _a_). When the setæ, which proceed
from the outer edge of {238} the auditory aperture and hide it, are
removed, it is seen to be a wide, somewhat triangular cleft, which
occupies the greater part of the hinder half of the tergal surface of
the basal joint (fig. 26, A).
The exopodites, or squames, of the antennæ extend as far as the apex
of the rostrum, or even project beyond it, when they are turned
forwards, while they reach to the commencement of the filament of
the endopodite (_Frontispiece_). The squame is fully twice as long
as it is broad, with a general convexity of its tergal and concavity
of its sternal surface. The outer edge is straight and thick, the
inner, which is fringed with long setæ, is convex and thin (fig. 48,
C). Where these two edges join in front, the squame is produced into
a strong spine. A thick outer portion of the squame is marked off
from the thinner inner portion by a longitudinal groove on the tergal
side, and by a strong ridge on the sternal side. One or two small
spines generally project from the posterior and external angle of the
squame; but they may be very small or absent in individual specimens.
Close beneath these, the outer angle of the next joint is produced
into a strong spine. When the abdomen is straightened out, if the
antennæ are turned back as far as they will go without damage, the
ends of their filaments usually reach the tergum of the third somite
of the abdomen (_Frontispiece_). I have not observed any difference
between the sexes in this respect.
The inner edge of the ischiopodite of the third {239} maxillipede
is strongly serrated and wider in front than behind (fig. 44); the
meropodite possesses four or five spines in the same region; and
there are one or two spines at the distal end of the carpopodite.
When straightened out, the maxillipedes extend as far as, or even
beyond, the end of the rostrum.
The inner or sternal edge of the ischiopodite of the forceps is
serrated; that of the meropodite presents two rows of spines, the
inner small and numerous, the outer large and few. There are several
strong spines at the anterior end of the outer or tergal face of this
joint. The carpopodite has two strong spines on its under or sternal
surface, while its sharp inner edge presents many strong spines. Its
upper surface is marked by a longitudinal depression, and is beset
with sharp tubercles. The length of the propodite, from its base to
the extremity of the fixed claw of the chela, measures rather more
than twice as much as the extreme breadth of its base, the thickness
of which is less than a third of this length (fig. 20, p. 93). The
external angular process, or fixed claw, is of the same length as the
base, or a little shorter. Its inner edge is sharp and spinose, and
the outer more rounded and simply tuberculated. The apex of the fixed
claw is produced into a slightly incurved spine. Its inner edge has a
sinuous curvature, convex posteriorly, concave anteriorly, and bears
a series of rounded tubercles, of which one near the summit of the
convexity, and one near the apex of the claw are the most prominent.
{240}
The apex of the dactylopodite, like that of the propodite, is
formed by a slightly incurved spine (fig. 20), while its outer,
sharper, edge presents a curvature, the inverse of that of the edge
of the fixed claw against which it is applied. This edge is beset
with rounded tubercles, the most prominent of which are one at the
beginning, and one at the end of the concave posterior moiety of the
edge. When the dactylopodite is brought up to the fixed claw, these
tubercles lie, one in front of and one behind the chief tubercle of
the convexity of the latter. The whole surface of the propodite and
dactylopodite is covered with minute elevations, those of the upper
surface being much more prominent than those of the lower surface.
The length of the fully extended forceps generally equals the
distance between the posterior margin of the orbit and the base of
the telson, in well characterized males; and, in individual examples,
they are even longer; while it may not be greater than the distance
between the orbit and the hinder edge of the fourth abdominal somite,
in females; and, in massiveness and strength, the difference of the
forceps in the two sexes is still more remarkable (fig. 2). Moreover
there is a good deal of variation in the form and size of the chelæ
in individual males. The right and left chelæ present no important
differences.
The ischiopodites of the four succeeding thoracic limbs are devoid
of any recurved spines in either sex (_Front._, fig. 46). The first
pair are the stoutest, the second the {241} longest: and when the
latter are spread out at right angles to the body, the distance from
tip to tip of the dactylopodites is equal to, or rather greater
than, the extreme length of the body from the apex of the rostrum to
the posterior edge of the telson, in both sexes. In both sexes, the
length of the swimmerets hardly exceeds half the transverse diameter
of the somites to which they are attached.
The exopodites of the appendages of the sixth abdominal somite (the
extreme length of which is rather greater than that of the telson)
are divided into a larger proximal, and a smaller distal portion
(fig. 37, F, p. 144). The latter is about half as long as the former,
and has a rounded free edge, setose like that of the telson. There
is a complete flexible hinge between the two portions, and the
overlapping free edge of the proximal portion, which is slightly
concave, is beset with conical spines, the outermost of which are
the longest. The endopodite has a spine at the junction of its outer
straight edge with the terminal setose convex edge. A faintly marked
longitudinal median ridge, or keel, ends close to the margin in a
minute spine. The tergal distal edge of the protopodite is deeply
bilobed, and the inner lobe ends in two spines, while the outer,
shorter and broader lobe, is minutely serrated.
In addition to the characters distinctive of sex, which have
already been fully detailed (pp. 7, 20, and 145), there is a marked
difference in the form of the sterna of the three posterior thoracic
somites between the males and females. {242} Comparing a male and
a female of the same size, the triangular area between the bases of
the penultimate and ante-penultimate thoracic limbs is considerably
broader at the base in the female. In both sexes, the hinder part of
the penultimate sternum is a rounded transverse ridge separated by a
groove from the anterior part; but this ridge is much larger and more
prominent in the female than in the male, and it is often obscurely
divided into two lobes by a median depression. Moreover, there are
but few setæ on this region in the female; while, in the male, the
setæ are long and numerous.
The sternum of the last thoracic somite of the female is divided by a
transverse groove into two parts, of which the posterior, viewed from
the sternal aspect, has the form of a transverse elongated ridge,
which narrows to each end, is moderately convex in the middle, and
is almost free from setæ. In the male, the corresponding posterior
division of the last thoracic sternum is produced downwards and
forwards into a rounded eminence which gives attachment to a sort of
brush of long setæ (fig. 35, p. 136).
The importance of this long enumeration of minute details[14] will
appear by and by. It is simply a statement of the more obvious
external characters in which all the full-grown English crayfishes
which have come under my {243} notice agree. No one of these
individual crayfishes was exactly like the other; and to give an
account of any single crayfish as it existed in nature, its special
peculiarities must be added to the list of characters given above;
which, considered together with the facts of structure discussed
in previous chapters, constitutes a definition, or diagnosis, of
the English kind, or _species_, of crayfish. It follows that the
species, regarded as the sum of the morphological characters in
question and nothing else, does not exist in nature; but that it is
an abstraction, obtained by separating the structural characters in
which the actual existences—the individual crayfishes—agree, from
those in which they differ, and neglecting the latter.
[14] The student of systematic zoology will find the comparison
of a lobster with a crayfish in all the points mentioned to be
an excellent training of the faculty of observation.
A diagram, embodying the totality of the structural characters
thus determined by observation to be common to all our crayfishes,
might be constructed; and it would be a picture of nothing which
ever existed in nature; though it would serve as a very complete
plan of the structure of all the crayfishes which are to be found
in this country. The morphological definition of a species is, in
fact, nothing but a description of the plan of structure which
characterises all the individuals of that species.
* * * * *
California is separated from these islands by a third of the
circumference of the globe, one-half of the interval being occupied
by the broad North Atlantic ocean. The fresh waters of California,
however, contain crayfishes which are {244} so like our own, that
it is necessary to compare the two in every point mentioned in
the foregoing description in order to estimate the value of the
differences which they present. Thus, to take one of the kinds of
crayfishes found in California, which has been called _Astacus
nigrescens_; the general structure of the animal may be described
in precisely the same terms as those used for the English crayfish.
Even the branchiæ present no important difference, except that the
rudimentary pleurobranchiæ are rather more conspicuous; and that
there is a third small one, in front of the two which correspond with
those possessed by the English crayfish.
The Californian crayfish is larger and somewhat differently coloured,
the undersides of the forceps particularly presenting a red hue.
The limbs, and especially the forceps of the males, are relatively
longer; the chelæ of the forceps have more slender proportions; the
areola is narrower relatively to the transverse diameter of the
carapace (fig. 61, C). More definite distinctions are to be found in
the rostrum, which is almost parallel-sided for two-thirds of its
length, then gives off two strong lateral spines and suddenly narrows
to its apex. Behind these spines, the raised lateral edges of the
rostrum present five or six other spines which diminish in size from
before backwards. The postorbital spine is very prominent, but the
ridge is represented, in front, by the base of this spine, which is
slightly grooved; and behind, by a distinct spine which is not so
strong as the postorbital spine. {245} There are no cervical spines,
and the middle part of the cervical groove is angulated backwards
instead of being transverse.
[Illustration: FIG. 62. A & D, _Astacus torrentium_; B & E, _A.
nobilis_; C & F, _A. nigrescens_. A–C, 1st abdominal appendage of the
male; D–F, endopodite of second appendage (× 3). _a_, anterior, and
_b_, posterior rolled edge; _c_, _d_, _e_, corresponding parts of the
appendages in each species; _f_, rolled plate of endopodite; _g_,
terminal division of endopodite.]
The abdominal pleura are narrow, equal-sided, and acutely pointed in
the males (fig. 61, F)—slightly broader, more obtuse, and with the
anterior edges {246} rather more convex than the posterior, in the
females. The tergal surface of the telson is not divided into two
parts by a suture (fig. 61, I). The anterior process of the epistoma
is of a broad rhomboidal shape, and there are no distinct lateral
spines.
The squame of the antenna is not so broad relatively to its length;
its inner edge is less convex, and its outer edge is slightly
concave; the outer basal angle is sharp but not produced into a
spine. The opposed edges of the fixed and movable claws of the
chelæ of the forceps are almost straight and present no conspicuous
tubercles. In the males, the forceps are vastly larger than in the
females, and the two claws of the chelæ are bowed out, so that a
wide interval is left when their apices are applied together; in the
females, the claws are straight and the edges fit together, leaving
no interval. Both the upper and the under surfaces of the claws
are almost smooth. The median ridge of the endopodite of the sixth
abdominal appendage is more marked, and ends close to the margin in a
small prominent spine.
In the females, the posterior division of the sternum of the
penultimate thoracic somite is prominent and deeply bilobed; and
there are some small differences in form in the abdominal appendages
of the males. More especially, the rolled inner process of the
endopodite of the second appendage (fig. 62 F, _f_) is disposed very
obliquely, and its open mouth is on a level with the base of the
jointed part of the endopodite (_g_) instead of reaching almost to
{247} the free end of the latter and being nearly parallel with
it. In the first appendage (C), the anterior rolled edge (_a_)
more closely embraces the posterior (_b_), and the groove is more
completely converted into a tube.
* * * * *
It will be observed that the differences between the English and
the Californian crayfishes amount to exceedingly little; but, on
the assumption that these differences are constant, and that no
transitional forms between the English and the Californian crayfishes
are to be met with, the individuals which present the characteristic
peculiarities of the latter are said to form a distinct species,
_Astacus nigrescens_; and the definition of that species is, like
that of the English species, a morphological abstraction, embodying
an account of the plan of that species, so far as it is distinct from
that of other crayfishes.
We shall see by and by that there are sundry other kinds of
crayfishes, which differ no more from the English or the Californian
kinds, than these do from one another; and, therefore, they are all
grouped as species of the one genus, _Astacus_.
[Illustration: FIG. 63. _Cambarus Clarkii_, male (1/2 nat. size),
after Hagen.]
If, leaving California, we cross the Rocky Mountains and enter the
eastern States of the North American Union, many sorts of crayfishes,
which would at once be recognised as such by any English visitor,
will be found to be abundant. But on careful examination it will be
discovered that all of these differ, both from the English crayfish,
and from _Astacus nigrescens_, to a much greater {248} extent
than those do from one another. The gills are, in fact, reduced
to seventeen on each side, in consequence of the absence of the
pleuro-branchia of the last thoracic somite; and there are some other
differences to which it is not needful to refer at present. It is
convenient to {249} distinguish these seventeen-gilled crayfishes,
as a whole, from the eighteen-gilled species; and this is effected by
changing the generic name. They are no longer called _Astacus_, but
_Cambarus_ (fig. 63).
All the individual crayfish referred to thus far, therefore, have
been sorted out, first into the groups termed _species_; and then
these species have been further sorted into two divisions, termed
_genera_. Each genus is an abstraction, formed by summing up the
common characters of the species which it includes, just as each
species is an abstraction, composed of the common characters of the
individuals which belong to it; and the one has no more existence
in nature than the other. The definition of the genus is simply a
statement of the plan of structure which is common to all the species
included under that genus; just as the definition of the species is a
statement of the common plan of structure which runs throughout the
individuals which compose the species.
[Illustration: FIG. 64.—_Parastacus brasiliensis_ (1/2 nat. size).
From southern Brazil.]
Again, crayfishes are found in the fresh waters of the Southern
hemisphere; and almost the whole of what has been said respecting the
structure of the English crayfish applies to these; in other words,
their general plan is the same. But, in these southern crayfishes,
the podobranchiæ have no distinct lamina, and the first somite of
the abdomen is devoid of appendages in both sexes. The southern
crayfishes, like those of the Northern hemisphere, are divisible
into many species; and these species {250} are susceptible of
being grouped into six genera—_Astacoides_ (fig. 65), _Astacopsis_,
_Chæraps_, _Parastacus_ (fig. 64), _Engæus_, and _Paranephrops_—on
the same principle as that which has led to the grouping of the
Northern forms into two genera. But the same convenience which has
{252} led to the association of groups of similar species into
genera, has given rise to the combination of allied genera into
higher groups, which are termed _Families_. It is obvious that the
definition of a family, as a statement of the characters in which a
certain number of genera agree, is another morphological abstraction,
which stands in the same relation to generic, as generic do to
specific abstractions. Moreover, the definition of the family is a
statement of the plan of all the genera comprised in that family.
[Illustration: FIG. 65.—_Astacoides madagascarensis_ (2/3 nat. size).
From Madagascar.]
The family of the Northern crayfishes is termed _Potamobiidæ_; that
of the Southern crayfishes, _Parastacidæ_. But these two families
have in common all those structural characters which are special
to neither; and, carrying out the metaphorical nomenclature of the
zoologist a stage further, we may say that the two form a _Tribe_—the
definition of which describes the plan which is common to both
families.
[Illustration: FIG. 66.—Diagram of the morphological relations of the
Astacina.]
It may conduce to intelligibility if these results are put into a
graphic form. In fig. 66, A. is a diagram representing the plan
of an animal in which all the externally visible parts which are
found, more or less modified, in the natural objects which we call
individual crayfishes are roughly sketched. It represents the plan
of the tribe. B. is a diagram exhibiting such a modification of
A. as converts it into the plan common to the whole family of the
_Parastacidæ_. C. stands in the same relation to the _Potamobiidæ_.
If the scheme were thoroughly worked out, diagrams representing the
peculiarities of {254} form which characterize each of the genera
and species, would appear in the place of the names of the former, or
of the circles which represent the latter. All these figures would
represent abstractions—mental images which have no existence outside
the mind. Actual facts would begin with drawings of individual
animals, which we may suppose to occupy the place of the dots above
the upper line in the diagram.
That all crayfishes may be regarded as modifications of the common
plan A, is not an hypothesis, but a generalization obtained by
comparing together the observations made upon the structure of
individual crayfishes. It is simply a graphic method of representing
the facts which are commonly stated in the form of a definition of
the tribe of crayfishes, or _Astacina_.
This definition runs as follows:—
Multicellular animals provided with an alimentary canal and with a
chitinous cuticular exoskeleton; with a ganglionated central nervous
system traversed by the œsophagus; possessing a heart and branchial
respiratory organs.
The body is bilaterally symmetrical, and consists of twenty metameres
(or somites and their appendages), of which six are associated into
a head, eight into a thorax, and six into an abdomen. A telson is
attached to the last abdominal somite.
The somites of the abdominal region are all free, those of the head
and thorax, except the hindermost, which is {255} partially free,
are united into a cephalothorax, the tergal wall of which has the
form of a continuous carapace. The carapace is produced in front into
a rostrum, at the sides into branchiostegites.
The eyes are placed at the ends of movable stalks. The antennules
are terminated by two filaments. The exopodite of the antenna has
the form of a mobile scale. The mandible has a palp. The first and
second maxillæ are foliaceous; the second being provided with a
large scaphognathite. There are three pairs of maxillipedes, and the
endopodites of the third pair are narrow and elongated. The next pair
of thoracic appendages is much larger than the rest, and is chelate,
as are the two following pairs, which are slender ambulatory limbs.
The hindmost two pairs of thoracic appendages are ambulatory limbs,
like the foregoing, but not chelate. The abdominal appendages are
small swimmerets, except the sixth pair, which are very large, and
have the exopodite divided by a transverse joint.
All the crayfishes have a complex gastric armature. The seven
anterior thoracic limbs are provided with podobranchiæ, but the first
of these is always more or less completely reduced to an epipodite.
More or fewer arthrobranchiæ always exist. Pleurobranchiæ may be
present or absent.
In this tribe of _Astacina_ there are two families, the _Potamobiidæ_
and the _Parastacidæ_; and the definition of each of these families
is formed by superadding to the {256} definition of the tribe the
statement of the special peculiarities of the family.
Thus, the _Potamobiidæ_ are those _Astacina_ in which the
podobranchiæ of the second, fourth, fifth, and sixth thoracic
appendages are always provided with a plaited lamina, and that of
the first is an epipodite devoid of branchial filaments. The first
abdominal somite invariably bears appendages in the males, and
usually in both sexes. In the males these appendages are styliform,
and those of the second somite are always peculiarly modified. The
appendages of the four following somites are relatively small. The
telson is very generally divided by a transverse incomplete hinge.
None of the branchial filaments are terminated by hooks; nor are
any of the coxopoditic setæ, or the longer setæ of the podobranchiæ
hooked, though hooked tubercles occur on the stem and on the laminæ
of the latter. The coxopoditic setæ are always long and tortuous.
In the _Parastacidæ_, on the other hand, the podobranchiæ are
devoid of more than a rudiment of a lamina, though the stem may be
alate. The podobranchia of the first maxillipede has the form of
an epipodite; but, in almost all cases, it bears a certain number
of well developed branchial filaments. The first abdominal somite
possesses no appendages in either sex: and the appendages of the
four following somites are large. The telson is never divided by a
transverse hinge. More or fewer of the branchial filaments of the
{257} podobranchiæ are terminated by short hooked spines; and the
coxopoditic setæ, as well as those which beset the stems of the
podobranchiæ, have hooked apices.
The definitions of the genera would in like manner be given by adding
the distinctive characters of each genus to the definitions of the
family; and those of the species by adding its character to those
of the genus. But at present it is unnecessary to pursue this topic
further.
* * * * *
There are no other inhabitants of the fresh waters, or of the land,
which could be mistaken for crayfishes; but certain marine animals,
familiar to every one, are so strikingly similar to them, that one
of these was formerly included in the same genus, _Astacus_; while
another is very often known as the “Sea-crayfish.” These are the
“Common Lobster,” the “Norway Lobster,” and the “Rock Lobster” or
“Spiny Lobster.”
The common lobster (_Homarus vulgaris_, fig. 67) presents the
following distinctive characters. The last thoracic somite is firmly
adherent to the rest; the exopodite of the antenna is so small as to
appear like a mere movable scale; all the abdominal appendages are
well developed in both sexes; and, in the males, the two anterior
pairs are somewhat like those of the male _Astacus_, but less
modified.
[Illustration: FIG. 67. _Homarus vulgaris_ (1/3 nat. size).]
The principal difference from the _Astacina_ is exhibited by
the gills, of which there are twenty on each side; namely, six
podobranchiæ, ten arthrobranchiæ, and four {259} fully developed
pleurobranchiæ. Moreover, the branchial filaments of these gills
are much stiffer and more closely set than in most crayfishes. But
the most important distinction is presented by the podobranchiæ,
in which the stem is, as it were, completely split into two parts
longitudinally (as in fig. 68, B); one half (_ep_) corresponding with
the lamina of the crayfish gill, and the other (_pl_) with its plume.
Hence the base (_b_) of the podobranchia bears the gill in front;
while, behind, it is continued into a broad epipoditic plate (_ep_)
slightly folded upon itself longitudinally but not plaited, as in the
crayfish.
[Illustration: FIG. 68. Podobranchiæ of A, _Parastacus_; B,
_Nephrops_; C, _Palæmon_. A′, C′, transverse sections of A and C
respectively. _a_, point of attachment; _al_, wing-like expansion of
the stem; _b_, base; _br_, branchial filaments; _ep_, epipodite; _l_,
branchial laminæ; _pl_, plume; _st_, stem.]
[Illustration: FIG. 69. _Nephrops norvegicus_ (1/2 nat. size).]
The Norway Lobster (_Nephrops norvegicus_, fig. 69) {261} resembles
the lobster in those respects in which the latter differs from the
crayfishes: but the antennary squame is large; and, in addition, the
branchial plume of the podobranchia of the second maxillipede is very
small or absent, so that the total number of functional branchiæ is
reduced to nineteen on each side.
These two genera, _Homarus_ and _Nephrops_, therefore, represent a
family, _Homarina_, constructed upon the same common plan as the
crayfishes, but differing so far from the _Astacina_ in the structure
of the branchiæ and in some other points, that the distinction must
be expressed by putting them into a different tribe. It is obvious
that the special characteristics of the plan of the _Homarina_ give
it much more likeness to that of the _Potamobiidæ_ than to that of
the _Parastacidæ_.
The Rock Lobster (_Palinurus_, fig. 70) differs much more from the
crayfishes than either the common lobster or the Norway lobster does.
Thus, to refer only to the more important distinctions, the antennæ
are enormous; none of the five posterior pairs of thoracic limbs
are chelate, and the first pair are not so large in proportion to
the rest as in the crayfishes and lobsters. The posterior thoracic
sterna are very broad, not comparatively narrow, as in the foregoing
genera. There are no appendages to the first somite of the abdomen
in either sex. In this respect, it is curious to observe that, in
contradistinction from the _Homarina_, the Rock Lobsters are more
closely allied to the _Parastacidæ_ than to the _Potamobiidæ_. {263}
The gills are similar to those of the lobsters, but reach the
number of twenty-one on each side.
[Illustration: FIG. 70. _Palinurus vulgaris_ (about 1/4 nat. size).]
In their fundamental structure the rock lobsters agree with
the crayfishes; hence the plans of the two may be regarded as
modifications of a plan common to both. To this end, the only
considerable changes needful in the tribal plan of the crayfishes,
are the substitution of simple for chelate terminations to the middle
thoracic limbs and the suppression of the appendages of the first
somite of the abdomen.
Thus not only all the crayfishes, but all the lobsters and rock
lobsters, different as they are in appearance, size, and habits of
life, reveal to the morphologist unmistakable signs of a fundamental
unity of organization; each is a comparatively simple variation of
the general theme—the common plan.
Even the branchiæ, which vary so much in number in different members
of these groups, are constructed upon a uniform principle, and the
differences which they present are readily intelligible as the result
of various modifications of one and the same primitive arrangement.
In all, the gills are _trichobranchiæ_; that is, each gill is
somewhat like a bottle-brush, and presents a stem beset, more or
less closely, with many series of branchial filaments. The largest
number of complete branchiæ possessed by any of the _Potamobiidæ_,
_Parastacidæ_, _Homaridæ_, or _Palinuridæ_, is twenty-one on each
side; {264} and when this number is present, the total is made up of
the same numbers of podobranchiæ, arthrobranchiæ, and pleurobranchiæ
attached to corresponding somites. In _Palinurus_ and in the genus
_Astacopsis_ (which is one of the _Parastacidæ_), for example, there
are six podobranchiæ attached to the thoracic limbs from the second
to the seventh inclusively; five pairs of arthrobranchiæ are attached
to the interarticular membranes of the thoracic limbs from the third
to the seventh inclusively, and one to that of the second, making
eleven in all; while four pleurobranchiæ are fixed to the epimera of
the four hindmost thoracic somites. Moreover, in _Astacopsis_, the
epipodite of the first thoracic appendage (the first maxillipede)
bears branchial filaments, and is a sort of reduced gill.
These facts may be stated in a tabular form as follows:—
_The branchial formula of Astacopsis._
Somites and Arthrobranchiæ.
their Podobranchiæ. /───────^───────\ Pleuro-
Appendages. Anterior. Posterior. branchiæ.
VII. 0 (ep. r.) 0 0 0 = 0 (ep. r.)
VIII. 1 1 0 0 = 2
IX. 1 1 1 0 = 3
X. 1 1 1 0 = 3
XI. 1 1 1 1 = 4
XII. 1 1 1 1 = 4
XIII. 1 1 1 1 = 4
XIV. 0 0 0 1 = 1
── ── ── ── ──
6 + ep. r. + 6 + 5 + 4 = 21 + ep. r.
{265}
This tabular “branchial formula” exhibits at a glance not only the
total number of branchiæ, but that of each kind of branchia; and that
of all kinds connected with each somite; and it further indicates
that the podobranchia of the first thoracic somite has become so far
modified, that it is represented only by an epipodite, with branchial
filaments scattered upon its surface.
In _Palinurus_, these branchial filaments are absent and the
branchial formula therefore becomes—
Somites and Arthrobranchiæ.
their Podobranchiæ. /───────^───────\ Pleuro-
Appendages. Anterior. Posterior. branchiæ.
VII. 0 (ep.) 0 0 0 = 0 (ep.)
VIII. 1 1 0 0 = 2
IX. 1 1 1 0 = 3
X. 1 1 1 0 = 3
XI. 1 1 1 1 = 4
XII. 1 1 1 1 = 4
XIII. 1 1 1 1 = 4
XIV. 0 0 0 1 = 1
── ── ── ── ──
6 + ep. + 6 + 5 + 4 = 21 + ep.
In the lobster, the solitary arthrobranchia of the eighth somite
disappears, and the branchiæ are reduced to twenty on each side.
In _Astacus_, this branchia remains; but, in the English crayfish,
the most anterior of the pleurobranchiæ has vanished, and mere
rudiments of the two next remain. It has been mentioned that other
_Astaci_ present a rudiment of the first pleurobranchia. {266}
_The branchial formula of Astacus._
Somites and Arthrobranchiæ
their Podobranchiæ. /───────^───────\ Pleuro-
Appendages. Anterior. Posterior. branchiæ.
VII. 0 (ep.) 0 0 0 = 0 (ep.)
VIII. 1 1 0 0 = 2
IX. 1 1 1 0 = 3
X. 1 1 1 0 = 3
XI. 1 1 1 0 or _r_ = 3 or 3 + _r_
XII. 1 1 1 _r_ = 3 + _r_
XIII. 1 1 1 _r_ = 3 + _r_
XIV. 0 0 0 1 = 1
── ── ── ── ──
6 + ep. + 6 + 5 + 1 + 2 or 3_r_ = 18 + ep. + 2 or 3_r_.
In _Cambarus_, the number of the branchiæ is reduced to seventeen by
the disappearance of the last pleurobranchia; while, in _Astacoides_,
the process of reduction is carried so far, that only twelve complete
branchiæ are left, the rest being either represented by mere
rudiments, or disappearing altogether.
_The branchial formula of Astacoides._
Somites and Arthrobranchiæ
their Podobranchiæ. /───────^───────\ Pleuro-
Appendages. Anterior. Posterior. branchiæ.
VII. 0 (ep. r.) 0 0 0 = 0 (cp. r.)
VIII. 1 _r_ 0 0 = 1 + _r_
IX. 1 1 0 0 = 2
X. 1 1 _r_ 0 = 2 + _r_
XI. 1 1 _r_ 0 = 2 + _r_
XII. 1 1 _r_ 0 = 2 + _r_
XIII. 1 1 _r_ 0 = 2 + _r_
XIV. 0 0 0 1 = 1
6 + _ep. r_ 5 + _r_ + 0 + 4 _r_ + 1 = 12 + _ep. r._ + 5 _r_.
{267}
As these formulæ show, those trichobranchiate crustacea, which
possess fewer than twenty-one complete branchiæ on each side,
commonly present traces of the missing ones, either in the shape
of epipodites, as in the case of the podobranchiæ, or of minute
rudiments, in the case of the arthrobranchiæ and the pleurobranchiæ.
In the marine, prawn-like, genus Penæus (fig. 73, Chap. VI.), the
gills are curiously modified trichobranchiæ. The number of functional
branchiæ is, as in the lobster, twenty; but the study of their
disposition shows that the total is made up in a very different way.
_The branchial formula of Penæus._
Somites and Arthrobranchiæ.
their /───────^───────\ Pleuro-
Appendages. Podobranchiæ. Anterior. Posterior. branchiæ.
VII. 0 (ep.) 1 0 0 = 1 + ep.
VIII. 0 (ep.) 1 1 1 = 3 + ep.
IX. 0 (ep.) 1 1 1 = 3 + ep.
X. 0 (ep.) 1 1 1 = 3 + ep.
XI. 0 (ep.) 1 1 1 = 3 + ep.
XII. 0 (ep.) 1 1 1 = 3 + ep.
XIII. 0 1 1 1 = 3
XIV. 0 0 0 1 = 1
── ── ── ── ──
0 + 6 ep. + 7 + 6 + 7 = 20 + 6 ep.
This case is very interesting; for it shows that the whole of the
podobranchiæ may lose their branchial character, and be reduced
to epipodites, as is the case with the first in the crayfish and
lobster, and indeed in most of the forms under consideration. And
since all but one of the somites bear both arthrobranchiæ and
pleurobranchiæ, {268} the suggestion arises that each hypothetically
complete thoracic somite should possess four gills on each side,
giving the following
_Hypothetically complete branchial formula._
Somites and Arthrobranchiæ.
their Podobranchiæ. /───────^───────\ Pleuro-
Appendages. Anterior. Posterior. branchiæ.
VII. 1 1 1 1 = 4
VIII. 1 1 1 1 = 4
IX. 1 1 1 1 = 4
X. 1 1 1 1 = 4
XI. 1 1 1 1 = 4
XII. 1 1 1 1 = 4
XIII. 1 1 1 1 = 4
XIV. 1 1 1 1 = 4
── ── ── ── ──
8 + 8 + 8 + 8 = 32
Starting from this hypothetically complete branchial formula, we
may regard all the actual formulæ as produced from it by the more
or less complete suppression of the most anterior, or of the most
posterior branchiæ, or of both, in each series. In the case of the
podobranchiæ, the branchiæ are converted into epipodites; in that of
the other branchiæ, they become rudimentary, or disappear.
* * * * *
In general appearance a common prawn (_Palæmon_, fig. 71) is very
similar to a miniature lobster or crayfish. Nor does a closer
examination fail to reveal a complete fundamental likeness. The
number of the somites, and of the appendages, and their general
character and {269} disposition, are in fact the same. But, in the
prawn, the abdomen is much larger in proportion to the cephalothorax;
the basal scale, or expodite of the antenna, is much larger; the
external maxillipedes are longer, and differ less from the succeeding
thoracic appendages. The first pair of these, which answers to the
forceps of the crayfish, is chelate, but it is very slender; the
second pair, also chelate, is always larger than the first, and
is sometimes exceedingly {270} long and strong (fig. 71, B); the
remaining thoracic limbs are terminated by simple claws. The five
anterior abdominal somites are all provided with large swimmerets,
which are used like paddles, when the animal swims quietly; and, in
the males, the first pair is only slightly different from the rest.
The rostrum is very large, and strongly serrated.
[Illustration: FIG. 71. _Palæmon jamaicensis_ (about 5/7 nat. size).
A, female; B, fifth thoracic appendage of male.]
None of these differences from the crayfish, however, is so great, as
to prepare us for the remarkable change observable in the respiratory
organs. The total number of the gills is only eight. Of these, five
are large pleurobranchiæ, attached to the epimera of the five hinder
thoracic somites; two are arthrobranchiæ, fixed to the interarticular
membrane of the external maxillipede; and one, which is the only
complete podobranchia, belongs to the second maxillipede. The
podobranchiæ of the first and third maxillipedes are represented only
by small epipodites. The branchial formula therefore is:—
Somites and Arthrobranchiæ.
their Podobranchiæ. /───────^───────\ Pleuro-
Appendages. Anterior. Posterior. branchiæ.
VII. 0 (ep.) 0 0 0 = 0 (ep.)
VIII. 1 0 0 0 = 1
IX. 0 (ep.) 1 1 0 = 2 (ep.)
X. 0 0 0 1 = 1
XI. 0 0 0 1 = 1
XII. 0 0 0 1 = 1
XIII. 0 0 0 1 = 1
XIV. 0 0 0 1 = 1
── ── ── ── ──
1 + 2 ep. + 1 + 1 + 5 = 8 + 2 ep.
{271}
The prawn, in fact, presents us with an extreme case of that kind of
modification of the branchial system, of which _Penæus_ has furnished
a less complete example. The series of the podobranchiæ is reduced
almost to nothing, while the large pleurobranchiæ are the chief
organs of respiration.
But this is not the only difference. The prawn’s gills are not
brush-like, but are foliaceous. They are not _trichobranchiæ_, but
_phyllobranchiæ_; that is to say, the central stem of the branchia,
instead of being beset with numerous series of slender filaments,
bears only two rows of broad flat lamellæ (fig. 68, C, C′, _l_),
which are attached to opposite sides of the stem (C′, _s_), and
gradually diminish in size from the region of the stem by which it is
fixed, upwards and downwards. These lamellæ are superimposed closely
upon one another, like the leaves of a book; and the blood traversing
the numerous passages by which their substance is excavated, comes
into close relation with the currents of aerated water, which are
driven between the branchial leaflets by a respiratory mechanism of
the same nature as that of the crayfish.
Different as these phyllobranchiæ of the prawns are in appearance
from the trichobranchiæ of the preceding _Crustacea_, they are easily
reduced to the same type. For in the genus _Axius_, which is closely
allied to the lobsters, each branchial stem bears a single series
of filaments on its opposite sides; and if these biserial filaments
are supposed to widen out into broad leaflets, the transition from
{272} the trichobranchia to the phyllobranchia will be very easily
effected.
The shrimp (_Crangon_) also possesses phyllobranchiæ, and differs
from the prawn chiefly in the character of its locomotive and
prehensile thoracic limbs.
* * * * *
There are yet other very well-known marine animals, which, in common
appreciation, are always associated with the lobsters and crayfishes,
although the difference of general appearance is vastly greater than
in any of the cases which have yet been considered. These are the
Crabs.
In all the forms we have hitherto been considering, the abdomen is as
long as, or longer than, the cephalothorax, while its width is the
same, or but little less. The sixth somite has very large appendages,
which, together with the telson, make up a powerful tail-fin; and
the large abdomen is thus fitted for playing an important part in
locomotion.
Again, the length of the cephalothorax is much greater than its
width, and it is produced in front into a long rostrum. The bases
of the antennæ are freely movable, and they are provided with a
movable exopodite. Moreover, the eye-stalks are not inclosed in a
cavity or orbit, and the eyes themselves appear above and in front
of the antennules. The external maxillipedes are narrow, and their
endopodites are more or less leg-like.
[Illustration: FIG. 72. _Cancer pagerus_, male (1/3 nat. size). A,
dorsal view, with the abdomen extended; B, front view of “face.”
_as_, antennary sternum; _or_, orbit; _r_, rostrum; 1. eyestalk; 2.
antennule; 3. base of antenna; 3′, free portion of antenna.]
None of these statements apply to the crabs. In these {273} animals
the abdomen is short, flattened, and apt to escape immediate notice,
as it is habitually kept closely applied against the under surface
of the cephalothorax. It is {274} not used as a swimming organ; and
the sixth somite possesses no appendages whatever. The breadth of
the cephalothorax is often greater than its length, and there is no
prominent rostrum. In its place there is a truncated process (fig.
72, B, _r_), which sends down a vertical partition, and divides
from one another two cavities, in which the swollen basal joints
of the small antennules (_2_) are lodged. The outer boundary of
each of these cavities is formed by the basal part of the antenna
(_3_), which is firmly fixed to the edge of the carapace. There is
no exopoditic scale; and the free part of the antenna (_3′_) is very
small. The convex corneal surface of the eye appears outside the base
of the antenna, lodged in a sort of orbit (_or_), the inner margin
of which is formed by the base of the antenna, while the upper and
outer boundaries are constituted by the carapace. Thus, while in all
the preceding forms, the eye is situated nearest the middle line,
and is most forward, while the antennule lies outside and behind it,
and the antenna comes next; in the crab, the antennule occupies the
innermost place, the antenna comes next, and the eye appears to be
external to and behind the other two. But there is no real change in
the attachments of the eye-stalks. For if the antennule and the basal
joint of the antenna are removed, it will be seen that the base of
the eye-stalk is attached, as in the crayfish, close to the middle
line, on the inner side, and in front of the antennule. But it is
very long and extends outwards, behind the antennule and the an--tenna;
{275} its corneal surface alone being visible, as it projects into
the orbit.
Again, the ischiopodites of the external maxillipedes are expanded
into broad quadrate plates, which meet in the middle line, and close
over the other manducatory organs, like two folding-doors set in a
square doorway. Behind these there are great chelate forceps, as in
the crayfish; but the succeeding four pairs of ambulatory limbs are
terminated by simple claws.
When the abdomen is forcibly turned back, its sternal surface is
seen to be soft and membranous. There are no swimmerets; but, in the
female, the four anterior pairs of abdominal limbs are represented by
singular appendages, which give attachment to the eggs; while in the
males there are two pairs of styliform organs attached to the first
and second somites of the abdomen, which correspond with those of the
male crayfishes.
The ventral portions of the branchiostegites are sharply bent
inwards, and their edges are so closely applied throughout the
greater part of their length to the bases of the ambulatory limbs,
that no branchial cleft is left. In front of the bases of the
forceps, however, there is an elongated aperture, which can be
shut or opened by a sort of valve, connected with the external
maxillipede, which serves for the entrance of water into the
branchial cavity. The water employed in respiration, and kept in
constant motion by the action of the scaphognathite, is baled out
through two apertures, which {276} are separated from the foregoing
by the external maxillipedes, and lie at the sides of the quadrate
space in which these organs are set.
There are only nine gills on each side, and these, as in the
prawn and shrimp, are phyllobranchiæ. Seven of the branchiæ are
pyramidal in shape, and for the most part of large size. When the
branchiostegite is removed, they are seen lying close against its
inner walls, their apices converging towards its summit. The two
hindermost of these gills are pleurobranchiæ, the other five are
arthrobranchiæ. The two remaining gills are podobranchiæ, and
belong to the second and the third maxillipedes respectively. Each
is divided into a branchial and an epipoditic portion, the latter
having the form of a long curved blade. The branchial portion of the
podobranchia of the second maxillipede is long, and lies horizontally
under the bases of the four anterior arthrobranchiæ; while the
gill of the podobranchia of the third maxillipede is short and
triangular, and fits in between the bases of the second and the third
arthrobranchiæ. The epipodite of the third maxillipede is very long,
and its base furnishes the valve of the afferent aperture of the
branchial cavity, which has been mentioned above. The podobranchia of
the first maxillipede is represented only by a long curved epipoditic
blade, which can sweep over the outer surface of the gills, and
doubtless serves to keep them clear of foreign bodies. {277}
_The branchial formula of Cancer pagurus._
Somites and Arthrobranchiæ.
their Podobranchiæ. /───────^───────\ Pleurobranchiæ.
Appendages. Anterior. Posterior.
VII. 0 (ep.) 0 0 0 = 0
VIII. 1 1 0 0 = 2
IX. 1 1 1 0 = 3
X. 0 1 1 0 = 2
XI. 0 0 0 1 = 1
XII. 0 0 0 1 = 1
XIII. 0 0 0 0 = 0
XIV. 0 0 0 0 = 0
── ── ── ── ──
2 + ep. + 3 + 2 + 2 = 9 + ep.
It will be observed that the suppression of branchiæ has here taken
place in all the series, and at both the anterior and the posterior
ends of each. But the defect in total number is made up by the
increase of size, not of the pleurobranchiæ alone, as in the case
of the prawns, but of the arthrobranchiæ as well. At the same time
the whole apparatus has become more specialized and perfected as a
breathing organ. The close fitting of the edges of the carapace,
and the possibility of closing the inhalent and exhalent apertures,
render the crabs much more independent of actual immersion in water
than most of their congeners; and some of them habitually live on dry
land and breathe by means of the atmospheric air which they take into
and expel from their branchial cavities.
Notwithstanding all these wide departures from the structure and
habits of the crayfishes, however, attentive examination shows that
the plan of construction of the {278} crab is, in all fundamental
respects, the same as that of the crayfish. The body is made up of
the same number of somites. The appendages of the head and of the
thorax are identical in number, in function, and even in the general
pattern of their structure. But two pairs of abdominal appendages
in the female, and four pairs in the male, have disappeared. The
exopodites of the antennæ have vanished, and not even epipodites
remain to represent the podobranchiæ of the posterior five pairs
of thoracic limbs. The exceedingly elongated eye-stalks are turned
backwards and outwards, above the bases of the antennules and
the antennæ, and the bases of the latter have become united with
the edges of the carapace in front of them. In this manner the
extraordinary face, or _metope_ (fig. 72, B) of the crab results from
a simple modification of the arrangement of parts, every one of which
exists in the crayfish. The same common plan serves for both.
* * * * *
The foregoing illustrations are taken from a few of our commonest
and most easily obtainable _Crustacea_; but they amply suffice
to exemplify the manner in which the conception of a plan of
organization, common to a multitude of animals of extremely diverse
outward forms and habits, is forced upon us by mere comparative
anatomy.
Nothing would be easier, were the occasion fitting, than to extend
this method of comparison to the whole of the several thousand
species of crab-like, crayfish-like, or {279} prawn-like animals,
which, from the fact that they all have their eyes set upon
movable stalks, are termed the _Podophthalmia_, or stalk-eyed
_Crustacea_; and by arguments of similar force to prove that they
are all modifications of the same common plan. Not only so, but the
sand-hoppers of the sea-shore, the wood-lice of the land, and the
water-fleas or the monoculi of the ponds, nay, even such remote forms
as the barnacles which adhere to floating wood, and the acorn shells
which crowd every inch of rock on many of our coasts, reveal the same
fundamental organization. Further than this, the spiders and the
scorpions, the millipedes and the centipedes, and the multitudinous
legions of the insect world, show us, amid infinite diversity of
detail, nothing which is new in principle to any one who has mastered
the morphology of the crayfish.
Given a body divided into somites, each with a pair of appendages;
and given the power to modify those somites and their appendages in
strict accordance with the principles by which the common plan of
the _Podophthalmia_ is modified in the actually existing members of
that order; and the whole of the _Arthropoda_, which probably make
up two-thirds of the animal world, might readily be educed from one
primitive form.
And this conclusion is not merely speculative. As a matter of
observation, though the _Arthropoda_ are not all evolved from one
primitive form, in one sense of the words, yet they are in another.
For each can be traced {280} back in the course of its development
to an ovum, and that ovum gives rise to a blastoderm, from which the
parts of the embryo arise in a manner essentially similar to that in
which the young crayfish is developed.
Moreover, in a large proportion of the _Crustacea_, the embryo leaves
the egg under the form of a small oval body, termed a _Nauplius_
(fig. 73, D), provided with (usually) three pairs of appendages,
which play the part of swimming limbs, and with a median eye. Changes
of form accompanied by sheddings of the cuticle take place, in
virtue of which the larva passes into a new stage, when it is termed
a _Zoæa_ (C). In this, the three pairs of locomotive appendages
of the _Nauplius_ are metamorphosed into rudimentary antennules,
antennæ, and mandibles, while two or more pairs of anterior thoracic
appendages provided with exopodites and hence appearing bifurcated,
subserve locomotion. The abdomen has grown out and become a notable
feature of the Zoæa, but it has no appendages.
In some _Podophthalmia_, as in _Penæus_ (fig, 73), the young leaves
the egg as a Nauplius, and the Nauplius becomes a Zoæa. The hinder
thoracic appendages, each provided with an epipodite, appear; the
stalked eyes and the abdominal members are developed, and the larva
passes into what is sometimes called the _Mysis_ or _Schizopod_
stage. The adult state differs from this chiefly in the presence of
branchiæ and the rudimentary character of the exopodites of the five
posterior thoracic limbs. {281}
[Illustration: FIG. 73. _Penæus semisulcatus._ A, adult (after de
Haan. 1/2 nat. size); B, Zoæa, and C, less advanced Zoæa of a species
of _Penæus_. D, Nauplius. (B, C, and D, after Fritz Müller.)]
In the Opossum-shrimps (_Mysis_) the young does not leave the pouch
of the mother until it is fully {282} developed; and, in this case,
the _Nauplius_ state is passed through so rapidly and in so early and
imperfect a condition of the embryo, that it would not be recognized
except for the cuticle which is developed and is subsequently shed.
[Illustration: FIG. 74. _Cancer pagurus._ A, newly hatched Zoæa; B,
more advanced Zoæa; C, dorsal, and D, side view of Megalopa (after
Spence Bate). (The figures A and B are more magnified than C and D.)]
{283}
In the great majority of the _Podophthalmia_, the Nauplius stage
seems to be passed over without any such clear evidence of its
occurrence, and the young is set free as a Zoæa. In the lobsters,
which have, throughout life, a large abdomen provided with
swimmerets, the Zoæa, after going through a Mysis or Schizopod stage,
passes into the adult form.
In the crab, the young leaves the egg as a Zoæa (fig. 74, A and B).
But this is not followed by a Schizopod stage, inasmuch as the five
hinder pair of thoracic limbs are apparently, from the first, devoid
of exopodites. But the Zoæa, after it has acquired stalked eyes and
a complete set of thoracic and abdominal members, and has passed
into what is called the _Megalopa_ stage (fig. 74, C and D), suffers
a more complete metamorphosis. The carapace widens, the fore part
of the head is modified so as to bring about the formation of the
characteristic metope: and the abdomen, losing more or fewer of its
posterior appendages, takes up its final position under the thorax.
In the Zoæa state, those thoracic limbs which give rise to the
maxillipedes are provided with well-developed exopodites, and
in the free Mysis state all these limbs have exopodites. In the
Opossum-shrimps these persist throughout life; in _Penæus_, the
rudiments of them only remain; in the lobster, they disappear
altogether.
Thus, in these animals, there is no difficulty in demonstrating
that embryological uniformity of type of all the {284} limbs,
complete evidence of which was not furnished by the development of
the crayfish. In this crustacean, in fact, it would appear that the
process of development has undergone its maximum of abbreviation.
The embryo presents no distinct and independent Nauplius or Zoæa
stages, and, as in the crab, there is no Schizopod or Mysis stage.
The abdominal appendages are developed very early, and the new born
young, which resembles the Megalopa stage of the crab, differs only
in a few points from the adult animal.
* * * * *
Guided by comparative morphology, we are thus led to admit that the
whole of the _Arthropoda_ are connected by closer or more remote
degrees of affinity with the crayfish. If we were to study the perch
and the pond-snail with similar care, we should be led to analogous
conclusions. For the perch is related by similar gradations, in the
first place, with other fishes; then more remotely, with frogs and
newts, reptiles, birds, and mammals; or, in other words, with the
whole of the great division of the _Vertebrata_. The pond-snail, by
like reasoning upon analogous data, is connected with the _Mollusca_,
in all their innumerable kinds of slugs, shellfish, squids, and
cuttlefish. And, in each case, the study of development takes us back
to an egg as the primary condition of the animal, and to the process
of yelk division, the formation of a blastoderm, and the conversion
of that blastoderm into a more or less modified {285} gastrula, as
the early stages of development. The like is true of all the worms,
sea-urchins, starfishes, jellyfishes, polypes, and sponges; and it is
only in the minutest and simplest forms of animal life that the germ,
or representative of the ovum becomes metamorphosed into the adult
form without the preliminary process of division.
In the majority even of these _Protozoa_, the typical structure
of the nucleated cell is retained, and the whole animal is the
equivalent of a histological unit of one of the higher organisms.
An _Amœba_ is strictly comparable, morphologically, to one of the
corpuscles of the blood of the crayfish.
Thus, to exactly the same extent as it is legitimate to represent
all the crayfishes as modifications of the common astacine plan,
it is legitimate to represent all the multicellular animals as
modifications of the gastrula, and the gastrula itself as a
peculiarly disposed aggregate of cells; while the _Protozoa_ are such
cells either isolated, or otherwise aggregated.
It is easy to demonstrate that all plants are either cell aggregates,
or simple cells; and as it is impossible to draw any precise line
of demarcation, either physiological or morphological, between the
simplest plants, and the simplest of the _Protozoa_, it follows that
all forms of life are morphologically related to one another; and
that in whatever sense we say that the English and the Californian
crayfish are allied, in the same sense, though not to the same
degree, must we admit that all living things {286} are allied.
Given one of those protoplasmic bodies, of which we are unable to
say certainly whether it is animal or plant, and endow it with such
inherent capacities of self-modification as are manifested daily
under our eyes by developing ova, and we have a sufficient reason for
the existence of any plant, or of any animal.
This is the great result of comparative morphology; and it is
carefully to be noted that this result is not a speculation, but
a generalisation. The truths of anatomy and of embryology are
generalised statements of facts of experience; the question whether
an animal is more or less like another in its structure and in its
development, or not, is capable of being tested by observation; the
doctrine of the unity of organisation of plants and animals is simply
a mode of stating the conclusions drawn from experience. But, if it
is a just mode of stating these conclusions, then it is undoubtedly
conceivable that all plants and all animals may have been evolved
from a common physical basis of life, by processes similar to those
which we every day see at work in the evolution of individual animals
and plants from that foundation.
That which is conceivable, however, is by no means necessarily true;
and no amount of purely morphological evidence can suffice to prove
that the forms of life have come into existence in one way rather
than another.
There is a common plan among churches, no less than {287} among
crayfishes; nevertheless the churches have certainly not been
developed from a common ancestor, but have been built separately.
Whether the different kinds of crayfishes have been built separately,
is a problem we shall not be in a position to grapple with, until we
have considered a series of facts connected with them, which have not
yet been touched upon.
{288}