CHAPTER X.

CLASS OF CRUSTACEA.

767. CRUSTACEA are Articulated animals, respiring by branchia or external gills, or by the general surface, and possessing a circulating apparatus, and separate sexes. Crabs and Lobsters are the types of this group; but a great number of animals of a much less complicated structure, and of a different external form, are also classed with these; for as we descend the natural series formed by these animals, we see the same general plan of structure gradually modified and simplified. The lowest Crustacea are even so imperfect, that they can only exist, attached like parasites, on other animals: whence most Naturalists have placed them with the Intestinal Worms.

768. The tegumentary skeleton of Crustacea generally possesses a very considerable degree of firmness. It has nearly always a stony hardness; and indeed contains a very considerable proportion of carbonate of lime. We may look upon this solid envelope as a kind of epidermis; for beneath it we find a membrane like the true skin of higher animals; and at certain times it detaches itself and falls off, in the same manner as the epidermis of Reptiles separates itself from their bodies (§ 471), and as we have also seen the enveloping membrane of the larvæ of Insects renew itself several times. We can easily understand the necessity of this change, in animals whose whole bodies are inclosed in a solid case; which, not being able to grow like the interior parts, would oppose an invincible obstacle to their development, if it could not be thrown off, as soon as it has become too small to lodge them commodiously. Thus the Crustacea change their skin during the whole time of their growth; and it would seem that the greater part of these animals grow during their whole life. The manner in which

232

BODY OF CRUSTACEA.-DIVISION INTO SEGMENTS.

they free themselves from their old shell is exceedingly singular. In general they manage to get out of it without occasioning the least change in its form; and when they have quitted it, the whole surface of their body is already clothed in a new casing,— which, however, is still soft, and does not acquire its requisite solidity for some days.

769. The body of Crustacea is composed of a series of rings,

more or less distinct. Sometimes most of these segments are simply articulated one with the other, so as to admit of a considerable degree of motion, as in the Sandhopper; sometimes they are nearly all soldered together, and are only distinguished by furrows situated at their line of juncture ; lastly, in other cases, their union

is still more close, and it is only by analogy that we are led to consider the trunk, resulting from their juncture, as made up of several rings, rather than of one only. Hence result, as we can easily understand, very great differences in the form of these animals; and if we compare with each other a Woodlouse (Fig. 457), a Sandhopper (Fig. 456), and a Crab (Fig. 458), we might be at first led to be

as

FIG. 457.-ONISCUS, OR
WOODLOUSE.

lieve them formed according to types entirely different; but a deeper study of their structure shows, that the composition of their tegumentary skeleton is essentially the same, and that the differences lie almost entirely in this,that most of the rings being quite distinct and moveable in the Woodlouse, are grown together in the Crab, and that certain analogous parts do not present the same proportions in both animals. Thus in the Woodlouse and

Sandhopper we find a distinct head (c) followed by a thorax composed of seven rings similar to each other (t' t'), and each

BODY OF CRUSTACEA.-DIVISION INTO SEGMENTS.

233

ring provided with a pair of legs (p, pp,): and at the posterior part of the body we see an abdomen, also composed of seven seg

ments (as), whose size diminishes rapidly, but whose form is nearly the same as in the thorax. In the Crab (Fig. 458), on the contrary, the head is not separated from the thorax; and it forms, with the whole middle part of the

body, but a single

mass, covered by a

large solid buckler

Fig. 458.—Cancer Pagurus, Linn., with the tail of the called the carapace.

male, a; and of the female, b.

The abdomen, at first, escapes observation; for it is bent down under the thorax, and is of small size. Yet it is easy to show, that in the Crab, as in the Woodlouse, there are seven very recognisable thoracic rings, and that the carapace is not a new part introduced instead of the former, but merely the dorsal portion of one of the rings of the head, so extremely developed that it has encroached upon all the neighbouring rings.

770. In other animals of the same class, the general form of the body differs still more widely from those of which we have just spoken. Thus the Limnadiæ are inclosed between two oval shields, joined like the valves of an Oyster, and it is only after having raised this moveable cuirass, that we first perceive the annular structure of the body (Fig. 490); the Cypris (Fig. 495), which abounds in stagnant waters, presents a similar arrangement: but the rings of which its body is composed, are still more difficult to recognise. Lastly, we may advert to the Lernea, which at their adult age present the strangest forms; but which, in the earlier part of their existence, possess a regular annular structure (Figs. 501, 502). This comparative study of

234

LATERAL APPENDAGES OF CRUSTACEA.

the tegumentary skeleton is of great interest as a department of Philosophic Anatomy; one of whose most important branches has reference to those modifications, to which Nature subjects the same organic elements, in order to adapt them to various purposes, and to create different animals from analogous materials; but the limits we have assigned ourselves, do not permit us to dwell longer on this subject.

771. The lateral appendages of the different rings constituting the body, are in general very numerous, and present considerable differences in their conformation and uses ;-both when we consider those in the different parts of the same individual, and when we compare them in distinct species. Those of the first pair are generally subservient to the animal functions, and support the eyes or constitute the antennæ; the next surround the mouth, and serve for the prehension or division of the food; those of the middle part of the body constitute the legs for locomotion; and those which are placed behind have various uses, but are generally subservient to respiration or reproduc

FIG. 459.-PRAWN-as, antennæ of the first pair; ai, antennæ of the second pair ; 7, laminar appendage covering its base; r, rostrum, or frontal prolongation of the carapace; y, eyes; pm, external foot-jaw; p, first thoracic member; p', second thoracic member; fp, false legs, or swimming members, of the abdomen; n, tail-fin.

tion; lastly, this long series ordinarily terminates by one or several pairs of members, which serve as fins.

772. The head, or rather the cephalic portion of the body, carries the eyes, the antennæ, and the appendages of the mouth.

LATERAL APPENDAGES OF CRUSTACEA.

235

It is sometimes divided into several distinct rings, as in the Scilla; but in general there is no separation, and it is formed of a single piece, which seems to represent seven segments joined together. Sometimes it is moveable, and distinct from the thorax (Fig. 458); sometimes, on the contrary, it is joined to this second part of the body, which in its turn is composed of distinct rings, articulating with each other in some species, but in others united into a single mass.

773. Of the antennæ, there are nearly always two pair; and they are generally thread-like, and very much elongated (Fig. 459, as and ai). The legs originate by pairs from the different thoracic rings; and they often amount to seven pairs, as for example in the Woodlouse (Fig. 457), and Sand-hopper (Fig. 466); but in other instances, as we see in Crabs (Fig. 458) and Cray-fish (Fig. 471), their number is reduced to five pairs; those appendages, which in the former constituted the four anterior legs, being subjected in the latter to other uses, and transformed into organs of mastication (§ 775). There are also very great differences in their structure: in some Crustacea they are leaf-like, membranous, and fitted for swimming only (Fig. 490); in others they look like little columns jointed together, and adapted for walking only; in others, while yet remaining appropriate to this last kind of locomotion, they are destined to be used also as so many little spades for digging in the earth, and then they are enlarged and laminated at the end (Fig. 460); lastly, in others, they end in pincers, and thus become instruments of prehension, at the same time that they fulfil their ordinary functions in locomotion (Figs. 471 and 505). In swimming Crustacea, such as Crayfish, Lobsters, Prawns, &c. (Fig. 459), the abdomen generally attains a considerable development, and ends in a large fin, in such a manner as to become the principal agent in