Skeleton of the Perch.
A A, Dorsal Fins; B, Caudal; C, Anal; D, Ventral; E, Pectoral.
The fins are the most important auxiliary organs of locomotion in fishes. The dorsal, caudal, and anal fins serve by their vertical position to increase the extent of the rowing surface, and to maintain the animal's balance, while the pectoral and ventral fins, which must be considered as the representatives of the fore and hind limbs of other vertebrata, are, moreover, of great assistance in directing its movements. With the help of these organs, fishes can advance or retrograde, ascend or descend in the water as they please, and it is curious to observe how, alternately extending or contracting one fin or the other, they gracefully plough the liquid element in every direction.
It is no less wonderful how perfectly the size and texture of the fins corresponds with the habits and necessities of the different species of fishes. Those which traverse vast portions of the ocean, or have frequently to struggle against swelling waves, are furnished with large and strong fins, while these organs are soft in the species which confine themselves to greater depths, where the winds cease to disturb the waters.
From the great variety which is met with both in the number and position of the fins, they are also of the greatest use in the classification of fishes, and afford the naturalist many of the chief characters which serve to distinguish the several orders, families, genera, and species of these aquatic vertebrates.[M]
[M] Cuvier divides the fishes into:
I. Chondropterygii—Skeleton cartilaginous; fins supported by cartilaginous rays; and
II. Osteopterygii—Skeleton composed of true bone.