In the greater number of the Decapoda the flabelliform appendages of the maxillary or of the ambulatory extremities penetrate into the respiratory cavity, and by their rnotions sweep, as it were, or stroke the surface of the branchim. Some anatomists have even imagined that it was by their action that the water necessary to respi ration was renewed in the in,terior of the branch ial • cavities ;* but ,this is a mistake ; these appen dages have little or no influence upon the cur rent which is continually traversing the respi ratory antrum, and which is produced by the motions of the great valvular lamina, already described as belonging to the second pair of the maxillipedes, and situated in the efferent respiratory canal.
The very secondary part which the flabelli form appendages of the thoracic extremities play in the interior of the respiratory cavities, is of itself a sure indication of the indetermi nateness of their numbers and relations to the bronchial pyramids. Thus whilst in the Lobster and the nearly allied genera, these ap pendages, to the number of five on either side, belong to the four first pairs of ambulatory ex tremities and to the third of the maxillary pairs, and run from below upwards between the branchial fasciculi, we only find three pairs in the Brachyura, belonging exclusively to the maxillary extremities, and penetrating into the branchial cavities horizontally, two on the outer surface of the branchi and one between the inner surface of these organs and the flancs.
We said in beginning this article that the Crustacea, by their general conformation, were evidently adapted to a purely aquatic life; this proposition must only be understood as gene rally applicable to the class, because there are genera which form exceptions to it, in regard to which we have still a few words to add.
The Telphusim and some other families of Crustaceans have the power of emerging from the water, and of entering it again after a longer or shorter stay upon dry land. But this fact is to be explained by the smallness of the two openings by which each of the branchial cavities communicates with the exterior, by which means a very small amount of evaporation only takes place from them. The whole of the Crab tribe have, in a greater or less degree, the faculty of'the par ticular species mentioned, provided the air by which they are surrounded is saturated with moisture; because if they die asphyxiated when brought into the air under ordinary cir cumstances, it is principally because their bran chiee having become dry are thereby unfitted to accomplish their functions.
But there are other species which are re markable for the faculty they possess not only of living habitually out of water, but be cause they are infallibly drowned by being kept long immersed in that fluid—these are the Gecarcini or land-crabs. Many hypotheses were broached to afford an explanation of this phenomenon, when a careful study of the diffe rent forms under which the organs of respira tion present themselves in these different genera, led us to discover in the membrane which lines the walls of the respiratory cavities, modifica tions analogous to those which are observed among fishes ofthe family of the Acanthopterygia pharyngew labyrinthiformes, &c. Sometimes we found folds and lacunm capable of serving as reservoirs of a certain quantity of water ; sometimes, as in the Birgus, a spongy mem brane equally well calculated to store up the fluid necessary to keep the org,ans of respira tion in the state of humidity essentially neces sary to enable them to perform their functions. It is well known, too, that the Land-crabs of which we are now speaking, never remove far froni damp situations. Some naturalists are of opinion that the tegumentary membrane with which the bramchial cavity is invested, is also the seat of active respiration; M. Geoffroy St. Hilaire even goes so far as to regard the growths with which the surface of this inembrane is covered in the Birgus, as constituting a true lung.
It would appear, consequently, that it is owing to the activity of the function of aerial respiration in the Gecarcini, that these ani mals are drowned when plunged under water, although they be provided with branchix ; and it is owing to these organs being kept in a suitable state of humidity that these creatures owe, at least in part, their faculty of breathing air.
We have said above that the principal cause of the death of our ordinary Crustaceans exposed to the air is the drying up of their branchim ; but this is not the sole cause of the asphyxia they suffer ; it would seem that the collapse of the branchial lamell which takes place when these organs are not supported by the water, and the greatly diminished extent of surface thereby exposed to the oxygenated fluid, con tributes mainly to prevent aerial respiration from proving adequate to maintain life among the common aquatic Crustacea.