The tegumentary skeleton of the Crustacea consists, like the bony skeleton of the Vette brata, of a great number of distinct pieces, connected together by means of portions of the epidermic envelope which have not become hardened, in the same way as among the higher animals certain bones are connected by cartilages, the ossification of which is only accomplished in extreme old age. On the varieties vvhich these pieces present in their number, their form, their relations, &c. depend the differences that occur in the conformation of the solid frame-work, the anatomical study of which is now about to engage our atten tion.
The most prominent feature in the external skeleton of the Crustacea is common to the whole grand division of articulated animals, and consists in the division of this envelope into a series of segments or rings, connected in suc cession one with another, and supporting tu bular appendages, also divided into segments, and armnged endwise. This peculiar structure is met with among the whole of the Crustacea; but when the frame-work of these animals is examined more narrowly, variations are disco vered so extensive and so numerous, that the mind is ahnost led to regard it as consisting of elements essentially different. Yet this is not so; and in pursuing the study, aided by the means of investigation developed in the pro gress of the philosophy of the natural sciences, very opposite results are elicited,—results which are replete with interest and instruction in regard to the mysteries of nature in her creative energies.
Now these methods of investigation may be reduced to two :—the first, which studies crea tures at their full growth, after having ar ranged them according to the natural order which follows from the investigation of their organization : the -second, which studies each creature, but the more perfect in preference, in the series of successive evolutions which constitute the different phases of the em bryonic state and of extra-uterine life ; for it is a demonstrated fact that these two series, so distinct, so widely separated in appearance, are in reality connected by links so inti mate, that the one is, in certain respects, the permanent reproduction of the other, which is the continual repetition of the first in one and the same individual.
By studying in this relative or comparative manner the skeleton of the Crustacea, we suc ceed in reducing to common principles the mode of conformation, apparently so various, of this apparatus, in the different groups formed by these animals. A remarkable tendency to uniformity of composition is every where re cognizable, and all the varieties are explicable in a general way by the laws in conformity with which the development of these animals takes place.
During the period of embryonic life the body is seen becoming divided into rings more and more numerous, and more and more unlike one another. The same tendency to diversity in the organization is also found in the types of which the series of Crustaceans consists; and in both instances the differences are readily seen to depend on various modifications undergone by parts originally similar. It is farther referable to one of the most general laws of organiza tion, viz. the tendency which nature shows to perfect functions by subdividing the work to be done, and throwing it upon a greater number of special organs. And we observe, in fact, among the most inferior animals that the different seg ments into which the body is divided are so completely repetitions of one another, that they all act precisely in the satne manner ; they severally include the elements necessary to the display of the vitality distinctive of the entire system to which they belong, so that they may be dissevered without any fun ction whatsoever being therefore the less completely performed in either of the detached portions. Many Annelidans present instances of this uniformity of composition. As we rise, how ever, in the scale of beings the different seg ments of the body are foun'd to become more and more unlike, both as regards their func tions and their conformation.
This law is also visibly manifested among the Crustaceans, whether they be studied at the various epochs of their embryonic state or compared together, examples being selected from the different groups of which this portion of the animal series consists. In either case a well-marked tendency to subdivision of the physiological operations is conspicuous; and in proportion as the divers acts, the aggregate of which constitutes the life of the individual, become attached to a particular system or place, the parts to which different functions are apportioned, acquire forms more dissimilar and more appropriate to their peculiar uses. When we come to treat of the evolution of the embryo of the Crustacea we shall have occa sion to revert to this subject, but it is neces sary so far to hint at it in this place, inasmuch as the conclusions which have been mentioned will often supply us with means of explaining those difficulties that are encountered when we seek to render compamtive the study of the different constituent parts of the external ske leton of the articulated series of animals.