Fishes are essentially formed for swimming (Fig. 340), and all their members are adapted for this purpose. The anterior members, which correspond with the arms in man and the wings in birds, are attached to each side of the trunk, immediately behind the head, and form the pectoral fins. The posterior members occupy the lower surface of the body, and form the ventral fins. The latter, which are always over the ventral line, may be before, beneath, or behind the former. Fishes possess, besides, fins in odd numbers. The fins which erect themselves on the back are called the back or dorsal fins, those at the end of the tail are the caudal fins; finally, there is frequently another attached to the lower extremity of the body, which is called the anal fin. These fins are always nearly of the same structure, consisting generally of a fold of the skin, supported by slender, flexible, cartilaginous or osseous rays, connected by a thin membrane.
Fig. 340. Skeleton of the Common Perch.
a, the inter-maxillary bone; b, the maxillary bone; d, the gills; c, the under jaw; f, the inter-operculum; g g, the vertebral column; h, the pectoral fin; i, the ventral fin; k and l, the dorsal fins; m, the anal fin; n, the caudal fin.
The muscles which bind together the vertebral column are so much developed in fishes as well as others of the superior animals, that they constitute in them alone the principal part of the body. The caudal, dorsal, and anal fins act as outlying oars; the pectoral and ventral fins assist in progression, at the same time that they help to maintain the equilibrium of the animal and guide and direct its movements, which are generally astonishing from their rapidity.
Fig. 341. Swimming bladder of the Carp.
An organ, which belongs properly to fishes[15] (Fig. 341), and which is usually considered as their chief aid in swimming, is a large bladder situated within the body, between the dorsal spine and the abdomen. This is usually called the swimming bladder. According to the volume this bladder assumes, the animal can increase or diminish the specific gravity of its body; that is, it can remain in equilibrium or ascend or descend in the bosom of the waters; it is, moreover, remarked that it is very small in those species which swim at the bottom of the water, and Mr. Gosse says there is some reason for considering it to be the first rudimentary form of the air-breathing lung.
Fig. 342. Anatomy of the Carp.