16 FISHES OF THE VICINITY OF NEW YORK 



bearing the ventral fins with it until it acquires a firm attachment 

 to the under side of the pectoral arch. In other words the hind limbs of a 

 bass are fastened to the under side of his collar bones. This arrangement 

 greatly facilitates the quick turning and steering of the fish, and fishes of 

 this improved type now far outnumber all others. 



Fish are enabled to rise or sink at will in the water by means of a 

 swim bladder. Originally this was probably a kind of pouch on the side 

 of the gullet, and may perhaps have served to secrete some fluid of use 

 in digestion. In many primitive fishes (lung fishes, ganoids) the "swim 

 bladder" has become richly supplied with blood vessels and assists in 

 the process of oxidizing the blood. In the higher fishes, however, it 

 loses the blood vessels and becomes tough and elastic in structure; it 

 also loses its connection with the gullet and forms a closed bag filled with 

 gases from the body of the fish. By some means that is not yet well 

 understood the fish is able either to compress the air bladder and thus 

 squeeze his body into a smaller space thus decreasing his bulk and caus- 

 ing the body to sink in the water, or, by relaxing the air bladder and ex- 

 tending the sides of the body, the fish can increase his bulk and rise in 

 the water. 



The necessity of free motion through a dense yet unstable medium 

 has held the evolution of the fish's body-form to compact and conven- 

 tional lines. Similarly the buoyancy of water has made supporting 

 structures (as limbs) unnecessary, and by neutralizing gravity, done 

 away with need for lightness. 



Theoretically there is no limit to the size of a fish since its weight is 

 supported by the water. Practically, however, the difficulty of moving 

 through a dense medium and the still more practical difficulty of getting 

 enough to eat seem to impose pretty definite limits on the size of fishes, 

 and just as the little rodents — rats and mice — are the most numerous 

 of land mammals so the smaller fishes are the commonest denizens of the 

 deep. 



In an earlier geological epoch there were fishes as big as whales, 

 but these huge sharks died out and it is not at all improbable that they 

 died because they could not get enough to eat. 



In conclusion we have seen that in many ways a fish resembles a 

 self-directing, self-propelled torpedo, and if space were available we 

 might point out other curious and interesting features of this kind : how 

 the eyes are turned about in their sockets, how the inner ear includes 

 delicate instruments for indicating the position of the body and for keep- 



