In the Apus and the Nebalia we conti nue to find the rings of the thoracic and abdo minal portions of the tegumentary skeleton nearly equal in size and similar in form; but the cephalic section, instead of presenting the same conformation as these two portions of the body, constitutes superiorly an immense shield, which extends over the rings of the thomx and conceals them. This dorsal shield or buckler, which is denominated Carapace by zoologists, also occurs among the whole of the Podoph thalmians, and more than all besides conspires to give to these animals their distinguishing peculiarities of shape. Inquiries, of which it would be tedious to give a detailed account in this place, have led us to discover that the carapace of these Crustaceans is neither more nor less than the superior arc of the third or fourth cephalic ring, enormously developed, and which in attaining its large dimensions laps over and modifies the conformation of a greater or smaller number of the neighbouring rings.* In the generality of the Stomapods the cara pace does not quite cover and conceal the two first cephalic rings, which indeed continue dis tinct and moveable ; but in the whole of the Decapods these rings cohere with one ano ther and with the following ones, and unite more and more intimately under the carapace, which then covers the whole of the head as well as the thorax. In the Macroura the anterior extremity of the carapace only extends over the ophthalmic or first cephalic ring ; but in the Brachyum it bends around this ring so as to include it, and to go to unite underneath with the next segment. As we ascend in the series of Crustaceans, we observe the carapace en croaching more and more upon the thorax. In the Squillw the three last cephalic and three first thoracic rings are nearly lost by becoming blended with those to which the carapace be longs; they scarcely retain any mobility, and protected above by this shield, unite intimately, and remain imperfect in their tergal portions ; the four last rings of the thorax continue, on the contrary, free, and are in almost every particular similar to those of the abdomen. In the Mysis this union of the cephalic shield with the seg ments of the thoracic division of the tegumen tary skeleton is carried further, for there are not more than two of these rings which remain distinct. But it is in the Decapods that the carapace attains its greatest development, and that its influence upon the evolution of the thoracic segments is carried the farthest.
In these animals the framework of the body does not appear at first sight to consist of more than two portions, the one anterior, formed by the carapace, and representing the cephalic and thoracic segments conjoined ; the other poste rior, formed by the abdomen. In reality, the first fourteen rings of the body are covered by this enormous buckler, and are so intimately conjoined as to have lost all their mobility; the whole of the thoracic segments thus bidden below the carapace, are connected with it in their superior part, they are only joined with one another underneath wail laterally ; and their tergal parts having, in consequence of this, be come useless, are no longer to be found, being in some sort replaced by the great cephalic buckler; thus the whole of these rings, in con formity with this arrangement, are imperfect and open above.
Hitherto we have not been able to deter mine whether the carapace of the Podophthal mia is formed at the expense of the third or of the fourth ring of the tegumentary skeleton ; but we have the strongest reasons to conclude that this buckler is neither more nor less than the dorsal arc of one or other of these cephalic rings, and not of the two conjointly. In fact
we can here demonstrate a composition analo gous to that which we have already pointed out as characteristic of every arc, whether supe rior or inferior, of the different rings in their state of complete development, to wit, a tergal portion and two lateral or epimeral pieces. In following the embryo of the Itiver-crab in its progressive stages of development, Rathke* observed the carapace to be formed of three pieces, which at length became consolidated so as to form but one. In many of the Deca pods it is even easy to perceive this structure or composition in the carapace of adults, inas much as there exist lines marking the conjunc tion, and accurately indicating the respective limits of the different pieces of which this great dorsal plate is composed.
The general form of the carapace depends in great measure on the relative development of these different pieces ; in the Macroura the tergal portion of the carapace extends but a short way backwards, whilst the lateral or epimeral pieces reach as far as the begin ning- of the abdomen, and being no longer kept at a distance by the tergum, meet in the median line of the back, and are there con joined. In the Brachyura, on the contrary, the tergal portion is that which is especially deve loped, so that it constitutes the whole of the upper part of the carapace, whilst the lateral pieces, thrust outwards and underneath, only form a narrow band above the bases of the ex tremities.
It is also in consequence of modifications analogous to those on which the existence of the carapace depends, that in other Crustacea the tegumentary skeleton presents the most singular forms : thus among the Limmadia and the Cypris, the pieces which are analogous to the epimeral or lateral pieces of this cephalic buckler, acquire a great extension, whilst the tergal portion of the arc to which they belong continues rudimentary or proves entirely abor tive, so that they constitute two large valves covering the whole body of the animal, and bearing considerable resemblance to the shells of certain acephalous Mollusks. The dorsal lamin which in the Pandarus form appendices on the back similar to Elytra, and those which in the Anthostomata form a kind of sheath around the posterior part of the body, are also formed by the anomalous development of cer tain parts of both the dorsal and ventral arcs of the two posterior thoracic rings.
The inferior arcs of the thoracic rings of the tegumentary skeleton of the Decapoda, by their intimate union, form a kind of ventral shield, named sternal plastrum, upon which lines of conjunction indicate the respective limits of the greater number of the segments, as well as of the sternal and episternal pieces of which these are composed. In the Decapoda Macroura and Anomoura, this plastrum is in general very narrow, but in the Brachyura it is expanded to such a degree as frequently to con stitute a great and nearly circular disc. In the whole of-these Crustaceans, the lateral pieces of the thoracic rings are conjoined, like those of the inferior arc of the same segments, and form on either side of the middle portion of the body a septum which is covered by the cara pace, and which is known among anatomists under the name of the vault of the jlanes. In the Macroura this septum is nearly vertical, but in the Brachyura it is oblique, or even almost horizontal.