In the Lcemodipoda, the parts which perform the office of branchim are vesicular bodies formed by the flabelliform appendage of a certain number of the pairs of thoracic extre mities. In the Isopoda, finally, the locomotory extremities no longer serve for respiration, the function being committed to the five first pairs of abdominal extremities which are entirely devoted to it and cease to have any other uses. These extremities, which are designated under the name of false branchial limbs, consist of a cylindrical articulation, supporting two folia ceous, soft membranous laminm, vascular in a greater or less degree ; frequently, too, we perceive on their inner side a small appendage, which may be reaarded as analogous to the femur or stem of the other extremities, whilst the two lamina; of which mention has just been made, appear to represent the palp and the flabellum. In the greater number of Iso poda these organs are completely exterior, but in several (such as the Idotea) the last ring of the abdomen supplies them with a cavity, the entrance to which is closed by valves which constitute the two appendages of the same ring.
The Stomapoda which have already supplied us with an instance of the absence of deter minate organs of respiration, also exhibit something analogous to the transition state of this apparatus during the second period of the embryonic life of the Decapod. In the genus Cynthia the branchim are represented by a small membranous cylinder, attached by its middle to a peduncle, itself implanted upon the extremity of the basilar articulation of the five first pairs of abdominal extremities.
The third type of the respiratory apparatus specified above, is presented to us by other Stomapods, known under the names of Squilhe and Thysanopodw. In these creatures, in filet, we discover branchim properly so called, the structure of which is greatly complicated, more so even than in the Crustaceans at the very bead of the series ; still the respiratory apparatus as a whole is much less complete, for they are not included in a cavity, and float freely in the water which bathes the entire sur face of the body of the animal. In the SquilIce (fig.425) the branchia2 are attached to the basilar joint of the first five pairs of abdominal extremi ties, and each consists of a long cylindrical tube, upon one of the sides of which proceeds a series of small tubes disposed parallel to one another like the pipes of an organ and support ing in their turn a series of long cylindrical and very numerous tubes.* In the Thysanopoda the branchize also resemble plumes, but in stead of being situated on the abdomen, they are attached to the thoracic extremities.f Finally, the last or highest term of develop ment which we have mentioned in the River crab, is also presented to us by the entire order of Decapod Crustaceans. Not only is the func
tion of respiration thrown upon particular organs, created expressly for this purpose, in the whole of these animals, but further, the organs themselves are lodged and protected within especial cavities, and the renewal of the water necessary to their functions is secured by the action of distinct appendages belonging more particularly to the masticatory and loco motory apparatuses.
Let us now take a survey of the branchial cavity. It occupies (fig. 426) the lateral part of the thorax, and extends between the vault of the flancs and the lateral portion of the cara pace, from the base of the extremities all the way towards the dorsal aspect of the animal. As we have already said, it is formed by an in ternal fold of the common tegumentary mem brane, which, after having formed the vault of tbe flancs, re-descends towards the base of the extremities to become continuous with the carapace. The internal and inferior wall of this cavity is consequently formed by the vault of the flancs itself, and its external and superior wall by a membranous septum, which in the greater part of its extent is for .the most part connected with the corresponding portion of the carapace. This last part of the walls of the branchial cavity presents an epidermic layer of extreme thinness, but covering a thick and shaggy membrane, the texture of which is found to vary, as we shall see by-and-by.
The cavity thus formed communicates ex ternally by two passages, the one destined for the entrance, the other for the exit of the water necessary to respiration. The disposition of the efferent opening varies but little; that of the afferent orifice, on the contrary, presents great varieties in the different groups of which the class of Decapods is coniposed.
The efferent orifice always occupies the ante rior extremity of the' branchial cavity, and is continuous with a canal (d, fig. 426 andf,fig. 428) the parietes of which are formed su periorly by the epimeral pieces of the last ce phalic rings, and inferiorly by the pterygo stomian portions of the carapace (b, fig. 427).
This canal runs forwards, passes to the out side of the oral apparatus, and terminates in front of the mouth (g,fig. 428). In its interior there is a large valve, which is falling and rising continnally, as if it moved upon a pivot, and which. in this way occasions a }sapid current from behind forwards in the water with which the cavity is filled. This valvular apparatus is neither more nor less than the flabelliform appendage of the second pair of maxillipedes which acquire dimensions in relation with the importance of the new function they have here to perform (h, fig. 428).