In the long-tailed Decapoda, and in the greater number of Anomoura of the same family, the respiratory cavity is open along the whole extent of its inferior edge; the carapace is not applied accurately to the lower margin of the vault of the flanc, and it is by the empty space thus left above the base of all the extremities that the water makes its way to the branchia2. In the Brachyura the afferent orifice of the branchial cavity is more circumscribed, but varies in a still greater degree. In nearly all the Crustacea it exists almost immediately in front of the base of the first pair of ambulatory extremities, and con sists of a kind of cleft, of considerable breadth, which in this place occurs between the edge of the carapace and the thorax (a, fig. 427), and which is occupied by a prolongation of the ba silar joint of the external maxillary limb (d), disposed in such a manner as to close it com pletely or to open it at the desire of the ani mal. In the genus Dorippus a slight variety in the disposition of this opening is ob served ; here at first view it appears to be pierced directly in the pterrostomian portion .
of the carapace; but it is in reality formed by an empty space left between the edge of the dorsal shield and the base of the external maxillary limb ; only here, this space, in stead of presenting itself immediately in front of the base of the anterior extremities, is se parated from this by a prolongation of the carapace. In the genus Ranina the carapace is joined to the thorax above the whole of these limbs, so as to leave no opening in this situation for the passage of the water, and it is at the origin of the abdomen that the afferent opening of the branchial cavity occurs. Lastly, in the Leucosia, this cavity is in. like manner completely closed above the base of the extre mities, and it is by a conduit parallel to the efferent canal, and opening outwardly likewise in front of the mouth, that the water reaches the interior of the branchial cavity.
The branchiw contained in the two cavities, one on either side, whose conformation we haVe now described, are disposed along the vaults of the flancs. They are shaped like a quadran gular pyramid, the base being fixed by means of a peduncle to the inferior part of this vault or to the membrane which extends from its in ferior edge to the basilar articulation of the Corresponding limb ; some of them are even inserted into this articulation. Each of these organs consists of two large longitudinal vessels situated on the opposite edges of a transverse septum, which extends from the base to the apex of the branchia, and presents on each side a great number of lamel lar or cylindrical pro longations. Of these two principal vessels the external is the affe rent one, of which men +inn hne alrna AA, hoar, 4W11 111.40 U1.•
made in treating of the circulation and its organs ; the internal again is the efferent vessel ; the capillaries by which these two communicate run in the substance of the branchial lamelbe, situated on either side of the median septum.
In the whole of the Decapoda brachyura and anomoura, and in the greater number of the ma croura, the folds of the tegumentary membrane which constitutes each branchia, are in the form of very thin lamellaa, directed perpendi cularly to the axis of tbe pyramid, and lying one over another like the leaves of' a book. But in Crawfish, the Lobster, the Nethrops, the Palinuri, the Scyllari, and the Gebi, these lamellw are replaced by a multitude of small cylinders, attached by their base, and closely packed side by side, like the bristles of a brush.
The number of branchial pyramids varies greatly, especially in the Macroura ; at the- most it is twenty-two, as is the case in the Astacus and the most nearly allied species; in other macroura the number is eighteen, as in the Palinuri, Scyllari, Penex ; fifteen, as in the Gebi; twelve, as in the Pandalus ; ten, as in the Calianassx ; eight, as in the Palemons ; and even seven only, as in the Crangons, politi, Sergestes, &c. In the Anomoura the number also varies very much. In the Bra chyura we can almost always reckon nine branchize on each side of the body ; two of this number, however, being merely rudiment ary ; sometimes two or one of these last is entirely wanting; and there are even species in which the branchia, which usually .occu pies the antipenultimate ring of the thorax, is missing.
The mode in which these organs are placed varies in a like degree : in the Brachyura (fig. 426) the whole, with the exception of two rudi mentary branchiw, are arranged along one and the same line, and rest parallel to one another upon the vault of the flancs ; the two last rings of the thorax never support any, and of the two rings which correspond to the second and third pairs of extremities, each presents a single py ramid attached to a hole pierced in the epimeral piece near to its inferior edge (fig.384). The five branchiw, situated in front of these, are attached above the edge of the vault of the flancs, and with the exception of the first are connected two and two upon coin mon peduncles. Lastly, the two rudimentary branchiw which complete the series anteriorly, are arranged under the base of the preceding; and attached to the basilar articulation of the second and third pairs of maxillary extremi ties. In the Anomoura and the Macroura, the branchi are often found arranged in several ranks, and generally occur on the two last thoracic segments, as well as upon those that precede these (jig. 431).