BRANCH CHORD AT A; CLASS PISCES: THE FISHES 275 



hollow sac-like depressions, closed at the rear. In 

 cases each sac has two openings or nostrils. The sense 

 oj" hearing is not very keen. The ears are fluid-filled sacs 

 buried in the skull, and without external or (except in a 

 few cases) internal opening. Fishes are far more sensi- 

 tive to sudden jars or sudden movements than to any 

 sound. They possess what is generally believed to be a 

 special sense organ not found in other animals. This is 

 the lateral line which extends along the sides of the body 

 and which consists of a series of modified scales (each one 

 with a mucous channel) richly supplied with nerves. The 

 eyes jire usually large and conspicuous. They differ 

 mainly from the eyes of other vertebrates in their myopic 

 spherical crystalline lens, made necessary by the density 

 of the medium in which fishes live. There are usually no 

 eyelids, the skin of the body being continuous but trans- 

 parent over the eyes. Being near-sighted, fishes do not 

 discriminate readily among forms, their special senses 

 fitting them in general to distinguish motions of their 

 enemies or prey rather than to ascertain exactly the 

 nature of particular things. 



The colors of fishes are in general appearance protec- 

 tive. Thus most individuals are white on the belly, 

 mimicking the color of the sky to the enemy which 

 pursues them from below. Seen from above most of them 

 are greenish, like the water, or brownish gray and 

 mottled, like the bottom. Those thaf live on sand are 

 sand-colored, those on lava black, and those among rose- 

 red sea-weeds bright red. In many cases, especially 

 among kinds that are protected by their activity, brilliant 

 colors and showy markings are developed. This is 

 especially true among fishes of the coral reefs, though 

 species scarcely less brilliant are found among the darters 

 of our American brooks. 



Among fresh-water fishes bright colors, crimson, 



