450 COLLEGE ZOOLOGY 



and, like the sharks, attack animals larger than themselves; 

 others are herbivorous to a considerable extent, feeding on 

 seaweeds and other vegetation ; and still others act as scavengers 

 or swallow mud from which both living and dead organisms are 

 obtained. 



Fishes are ultimately dependent upon microscopic organisms, 

 as is illustrated by the following example : — 



" On the morning of July 23 there was taken a large specimen 

 (squeteague) whose stomach contained an adult herring. In 

 the stomach of the herring were found two young scup (besides 

 many small Crustacea), and in the stomach of one of these scup 

 were found copepods, while in the alimentary tract of these last, 

 one could identify one or two of the diatoms (unicellular plants) 

 and an infusorian test among the mass of triturated material 

 which formed its food." (Peck.) 



Most fishes possess teeth on the jaws, roof of the mouth, or 

 gill-arches. These are used principally for holding food, but 

 in some cases for mastication. The most primitive type of 

 tooth is a simple pointed cone. Some fishes have front teeth 

 for capturing prey and back teeth for crushing; and in others 

 the teeth are all modified for crushing. Teeth that are lost or 

 worn away are generally replaced. 



The alimentary canal is usually similar to that of the perch. 

 Gastric glands in the walls of the stomach secrete digestive 

 juices. The intestine often possesses blind pouches, the pyloric 

 caeca, which increase the secretory surface. 



The Circulatory System. — Circulation in fishes is essentially 

 like that already described (Fig. 361). Lymph spaces and lymph 

 capillaries are situated in various parts of the body ; they collect 

 blood plasma from the tissues and transport it to the veins. 



The body of the fish contains several ductless glands which 

 may be considered under the circulatory system. The functions 

 of these glands are not well known. The extirpation of them 

 results in serious disturbances, and, in some cases, death. They 

 secrete substances (internal secretions) directly into the blood 



