_Mollusca._—Among the mollusks, the classes are nearly all named from the peculiarities in form or the position of the feet. We have the barnacles (_Cirrhopoda_), animals with six pair of articulated flexible arms, composed of a series of small pieces studded with minute hairs, and which form, when they are extended, a species of net by means of which the animal catches its prey. The _Branchiopoda_—creatures such as the _Terebratula_—with two long fleshy arms placed near the mouth, which are hollow, muscular, and disposed in spiral folds. We have the _Tunicata_, such as _Ascidians_ and the _Conchifera_, as the oyster and mussel, beings which are destitute of members, and all but incapable of locomotion. The _Gasteropoda_, of which the slug and snail afford familiar examples, and that are furnished with a locomotive apparatus peculiar to the whole class, and termed the foot or ventral disc. The _Pteropoda_, nearly allied to the _Gasteropoda_ in their internal organisation, but differing from them in the possession of two broad fleshy expansions, resembling fins, which are attached to the side of the neck, and are well adapted for swimming. And, lastly, we have the _Cephalopoda_, such as the nautilus and cuttle-fish, creatures with numerous strong muscular arms attached around the head, and in some species provided with sucking discs, by means of which they are enabled successfully to attack and capture the strongest and most active prey. In all these instances we clearly see that the appendages characterise the class, and that, from an acquaintance with their number and arrangement, a naturalist could readily come to a conclusion as to the affinities, structure, and general habits of any animal in the series. _Fishes._—The fishes are divided into two great natural orders—those furnished with a bony skeleton and those in which the framework of the body is cartilaginous. The former division are distinguished by the possession of a swimming bladder, and by the nearly equal developement of the parts corresponding to the extremities—the pectoral and ventral fins. The pectoral fins, situated immediately behind the head, are present in all the osseous fishes, traces of them being found even where they might be supposed to be entirely wanting, as in the eel; they do not, however, attain any very considerable size, rarely equalling, and never surpassing, the magnitude of the ventral fins. In the cartilaginous fishes, on the contrary, not only in those which ordinarily remain quiet at the bottom of the water, as the skate, the sole, the flounder, and other flat fishes, but even in the active and voracious sharks we find no swimming-bladder. In these cases, its place is supplied and its duty performed by the enormously developed anterior member or pectoral fin, which greatly surpasses the ventral fin in size; and it is by means of the active movements of this large hand that these creatures raise themselves from the bottom of the water to the surface.