# The Crayfish (Huxley)/Chapter 5

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 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 develope 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 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 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. [note 1] The 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.
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.
[Figure 61: Astacus torrentium, A. nobilis, A. nigrescens, carapaces]
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 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 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 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.
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 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 run, 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 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. 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 cornea1 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 eve-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 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 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.
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 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. 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 [note 2] 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 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 time 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.
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 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. There are no cervical spines, and the middle part of the cervical groove is angulated backwards instead of being transverse.
The abdominal pleura are narrow, equal-sided, and acutely pointed in the males (fig. 61, F) -- slightly broader, more obtuse, and with time anterior edges 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.
[Figure 62: Astacus torrentium, A. nobilis, A. nigrescens, appendages]
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 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.
[Figure 63: Cambarus Clarkii]
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 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 pleurobranchia 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 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.
[Figure 64: Parastacus brasiliensis]
[Figure 65: Astacoides madagascarensis]
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 are susceptible of being grouped into six genera--Astacoides (fig. 65), Astacopsis, Chæraps, Parastacus (fig. 64), Eugæus, 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 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.
[Figure 66:--Diagram of the morphological relations of the Astacina.]
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.
It may conduce to intelligibility if these results arc 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 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 oesophagus; 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 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. Time 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 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 time 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 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."
[Figure 67: Homarus vulgaris]
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.
[Figure 68: Podobranchiæ of Parastacus, Nephrops, Palæmon]
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 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, m 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.
[Figure 69: Nephrops norvegicus]
The Norway Lobster (Nephrops norvegicus, fig. 69) 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.
[Figure 70: Palinurus vulgaris]
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. The gills are similar to those of the lobsters, but reach the number of twenty-one on each side.
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; 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 Podo-|------------|Pleuro-
IX. ... 1... 1... 1 ... 0 = 3
XI. ... 1... 1... 1 ... 1 = 4
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--
[The branchial formula of Palinurus]
Somites and Arthrobranchiæ
their Podo-|------------|Pleuro-
IX. ... 1... 1... 1 ... 0 = 3
XI. ... 1... 1... 1 ... 1 = 4
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.
The branchial formula of Astacus.
Somites and Arthrobranchiæ
their Podo-|------------|Pleuro-
IX. ... 1... 1... 1 ... 0 = 3
XI. ... 1... 1... 1 ... 0 or r = 4 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 Podo-|------------|Pleuro-
IX. ... 1... 1... 0 ... 0 = 2
XI. ... 1... 1... r ... 0 = 2 + r
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 Podo-|------------|Pleuro-
IX. ... 0 (ep.) ... 1... 1 ... 1 = 3 + ep.
XI. ... 0 (ep.) ... 1... 1 ... 1 = 3 + 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, 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 Podo-|------------|Pleuro-
IX. ... 1... 1... 1 ... 1 = 4
XI. ... 1... 1... 1 ... 1 = 4
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.
[Figure 71: Palæmon jamaicensis]
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 disposition, are in fact the same. But, in the prawn, the abdomen is much larger in proportion to the cephalothorax; the basal scale, or exopodite of the antenna, is much larger; the external maxillipedes are longer, and differless 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 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.
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
[The branchial formula of Palæmon]
Somites and Arthrobranchiæ
their Podo-|------------|Pleuro-
IX. ... 0 (ep.) ... 1... 1 ... 0 = 2 (ep.)
XI. ... 0... 0... 0 ... 1 = 1
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 stein 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 stein (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 ærated 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 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.
[Figure 72: Cancer pagurus]
None of these statements apply to the crabs. In these 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 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 antenna; 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 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.
The branchial formula of Cancer pagurus.
Somites and Arthrobranchiæ
their Podo-|------------|Pleuro-
IX. ... 1... 1... 1 ... 0 = 3
XI. ... 0... 0... 0 ... 1 = 1
It will be observed that the suppression of branchiæ has here taken place in all the series, and at both the anterior and time 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 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 time 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 eyestalks 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 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 time 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.
[Figure 73: Penæus semisulcatus]
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 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, bat 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.
In the Opossum-shrimps (Mysis) the young does not leave the pouch of the mother until it is fully 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.
[Figure 74: Cancer pangurus, Zoæa and Megalopa]
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 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 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 Amoeba 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 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 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.