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 ; 0, Anal ; d, Ventral , e. 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 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.* 



* Cuvier divides the fishes into : 



I. Chondropterygii — Skeleton cartilaginous; fins supported by cartilaginous 

 rays; and 



II. Osteopterygii — Skeleton composed of true bone. 



