By general consent and usage, three regions are recognized in the bodies of these animals,— a head, a thorax, and an abdomen ; and from this custom we shall not depart, although we must avow that these denominations are only derived from very clumsy views, and are calculated to convey false impressions in regard to the nature and composition of the parts so named, by leading the mind to liken them to the grand divisions entitled head, thorax, and abdomen in the Vertebrata. Nevertheless, with the ex ception of the objectionable names, the division of the body into three regions is not less a fact as regards the organization of the Crustaceans ; and the one-and-twenty rings of which, as we have said, their body consists in the type to which every member of the class may be re ferred, are generally found divided into three equal series of seven, each of which may be held as corresponding with one of the three regions. This law of composition is observed to obtain not only among the more simple species, where the rings generally resem ble each other most closely, but its influence may be remarked among the most complicated also, and amidst exceptions and contradictions in appearance the most obvious. The head or cephalic region includes the principal organs of sense as among the Vertebrata, the com mencement of the apparatus subservient to digestion, and the appendages destined to seize and masticate the food. The thorax, strictly speaking, forms no cavity distinct from the pre ceding, but is its continuation; the part espe cially designated thorax, however, is that which is included from front to back between the head and the beginning of the abdomen, and is formed by the rings to which the extremities serving for locomotion are attached. This mid dle portion of the general cavity of the body contains almost the whole of the viscera. As to the abdomen, it succeeds the last of the thoracic rings, distinguishable by the presence in it of the orifices of the male organs of gene ration ; the appendices attached to it do not commonly attain any considerable size, and do not serve in a general way as organs of locomo tion ; to conclude, nothing is found in its inte rior save muscles and the terminal portion of the intestinal canal, the anal orifice of which exists in the last of the abdominal series of rings.
These three portions of the tegumentary ske leton are not always equally distinct, and their respective limits may even vary, for we occa sionally observe two or three of the foremost thoracic rings detaching themselves, as it were, from this region to which they properly belong, to join or blend with the cephalic rings ; and the same thing may be said in regard to the segments of which each of the remaining divi sions of the body consists; we in fact know of no specimen of a Crustacean in which the whole of the rings are moveable upon one another ; a certain number of them always appear to be come consolidated, and this union is frequently so intimate that all traces of its existence are obliterated, so that the section of the body which results from this aggregation of rings appears to consist of no more than a single piece, and on a cursory view might be held to be constituted by a simple ring. The shape and size of these compound rinas varies also, circumstances which evidently depend on the unequal development of the different pieces of which they severally consist.
This consolidation of the rings occurs with increasing frequency as we rise in the scale of Crustaceans, and approach those the organiza tion of which is most complex ; yet there are a considerable number of species which form ex ceptions to this rule. The consolidation of the rings also shows a tendency to take place in the same order in which the different segments of the tegumentary skeleton appear in the em bryo, that is to say from before backwards : thus it is generally complete as regards the cephalic rings ; it is more frequent as regards the foremost than the hindmost thoracic rings ; and it but rarely occurs among the abdominal rings.
The differences which present themselves in the dimensions and forms of the different rings of the tegumentary skeleton, and which concur so essentially in producing varieties in the ge neral form of the Crustaceans, also show a ten dency to become greater and greater as we as cend in the series of these animals, and com monly influence the cephalic rings in a degree greater than those of the divisions situated more posteriorly.
To conclude, it is also among the most ele vated Crustaceans that the tegumentary skeleton is complicated in the greatest degree by the evolution of apodemata in the interior of the rings ; and further, it is in the cephalo-thoracic portion of the skeleton only that these laminw are encountered.
A few examples will render these general rules more readily appreciated.
In the earlier periods of evolution of the embryo of the river-crab, the whole of the rings, which are even then apparent, are of the. same form and dimensions, and the segments, which only appear at a later date, are a.t first similar to what these rings were in the beginning. This state of uniformity in the composition of the whole of the constituent rings of the tegumentary skeleton, which is invariably tran sient in the embryo, is not observed as a permanent feature in any perfectly developed Crustacean ; still there are several of these ani mals which are but little removed from it. In the Branchipods, for instance, the body consists of a long series of rings, having, with the ex ception of the very first, as nearly as possible the same form and the same dimensions. In the Amphipods (fig. 382) the want of resemblance between the different rings of the body becomes much more remarkable: the first seven become so completely united that they form a single piece in which no trace even of the lines of consolidation remains, and the conical segment which constitutes the head grows much more slowly than the rest of the body, so that the re lative dimensions become smaller and smaller as regards the head in proportion as the animal approaches the adult age. The seven rings of the thorax, on the other hand, continue per fectly distinct, and differ but little from one another; and the seven abdominal rings, in like manner, remain moveable, and only differ from those of the thorax as they do from one another by a relatively inferior degree of deve lopment. In the majority of the Isopods the structure of the tegumentary skeleton is essen tially the same as in the Amphipods ; but there occurs a greater inequality of development be tween the thoracic and the abdominal rings, most of the latter remaining more or less in a rudimentary state.