_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.