188 THE INHABITANTS OF THE SEA. 
The fins are the most important auxiliary organs of locomo- 
tion 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 
Skeleton of the Perch. 
A A, Dorsal Fins ; B, Caudal; c, Anal; p, Ventral; 8, Pectoral. 
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 mcvements. 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 aud 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.* 
* Cuvier divides the fishes into: 
I. Chondropterygii—Skeleton cartilaginous; fins supported by cartilaginous 
rays; and 
II. Osteopterygii— Skeleton composed of true bone. 
