152 



DIGESTION IK THE LOWER ANIMAI.S 



larger number of body cells, the character of the food 

 required by these cells, the opportunities for getting food, 

 and many other factors combine to produce these greater 

 modifications of structure. The nature of these adapta- 



FiG. 53 — Section of a fish ; st, stomach ; i, intestine ; g, gills. Davison, Zoology. 



tions to increased needs, however, is not different in 

 kind from that indicated in the earthworm. A few 

 types 'will ilhistrate the direction which these modifica- 

 tions have taken in different animals. 



Modifications in the digestive tube of the fish. — In this 

 animal, the skin that lines the mouth has produced out- 

 growths of a horny character called teeth, which prevent 

 the food from slipping backward when seized. These 

 are not like orn- teeth in that they are not set in sockets, 

 and are found all over the surface of the mouth. In the 

 mouth also, is found a fleshy motionless projection called- 

 the tongue, which has in it nerves of taste. It has no 

 movement and does not aid in swallowing like ours. Back 

 of the mouth is a widened space or pharynx, in the sides 

 of which four slits are developed for letting in water to 

 the breathing organs or gills. From the phar3mx the 



