The frame-work or solid parts of the Crus tacea consist, a,s we have said, of a series of rings.
The number of these rings may vary, but this happens to a much less extent than on a superficial view we might be led to conclude. By calling in to our aid the principles of ob servation and of comparison pointed out above, we have found that in every member of this class of animals the normal number of seg ments of the body is ti,venty-one. But a very few instances of a larger number oc curring are known, and it seldom happens that the number falls short of that which has been indicated. Occasionally, it is true, one or more rings prove abortive, and are never developed ; but in general their apparent ab sence depends entirely on their intimate union one with another, and other obvious indica tions of their existence may be discovered. By-and-by we shall find that in the embryo these segments are formed in succession from before backwards, so that, when their evolution is checked, the later rather than the earlier rings are those that are wanting; and in fact it is generally easy to see in those specimens of full-grown crustaceous animals whose bodies present fewer than twenty segments, that the anomaly depends on the absence of a certain number of the most posterior rings of the body.
The Lcemodipods, the Entomostraea, and the llaustellate Crustacea present us with instances of this condition, which calls to mind one of the stages through which the embryo of the higher species, whose development is the most complete, is known to pass.
Each segment of the body, when it attains its normal condition, consists of two distinct ele ments : the central or annular portion, and cer tain appendices which it supports.
The central or annular portion of the seg ments of the tegumentary skeleton presents, in its most simple state, the appearance of a com plete ring, but instead of a single piece it is requisite to count in its composition no fewer than eight, as has been demonstrated by the inquiries of M. Audouin on the structure of the thorax of insects,*' inquiries the results of which are immediately and almost wholly applicable to the Crustacea so nearly allied to the insects in their organization. Each ring is divided
first into two arcs, the one superior or dorsal, the other inferior or ventral, and each arc may present as many as four elementary pieces. Two of these pieces by being united in the me as to appear but one ; yet the comparative study of the apparatus in the different members of the class at large, leaves no doubt of their existence severally.
It frequently happens that the tegumentary membrane is folded so as to penetrate more or less deeply the interior of the ring among the different organs which fill the cavity. These folds, which may become solid lamin by being impregnated with calcareous salts, have received the name of apodemata, and always proceed from the lines of conjunction of the different pieces, or of the different rings with one another. We shall have occasion to revert to this part of our subject very shortly.
dian line constitute the tergum (fig. 378, D) ; the superior arc is completed on either side by two other pieces, known under the name dflancs or epimeral pieces (fig. 378, e). The inferior arc presents in its composition an exact counter part of the superior. Two of the four pieces into which it may be resolved constitute the sternum, situated in the median line, and are flanked by the two episternums. The two arcs thus composed, instead of cohering by their edges, leave a space for the insertion of the lateral appendages or extremities which corre spond with them. It is true, indeed, that we have no instance of any single ring which exhi bits the whole of these pieces distinct from one another ; in general several are anchylosed so The ,structure of the ring once investigated in the manner we have done, let us now pro ceed to inquire in what manner the different rings by the modifications they undergo, and by the divers modes of union they present, give rise to the variety of forms we observe among the Crustaceans.