170 



COLLEGE ZOOLOGY 



Tnternal Anatomy. — If an 

 animal is cut open along the 

 dorsal line (Fig. in), it will be 

 found to contain a straight ali- 

 mentary canal, and certain other 

 organs, lying in a central cavity, 

 the coelom. The alimentary canal 

 (2) is very simple, since the food 

 is taken from material already 

 digested by the host whose in- 

 testine the worm inhabits. It 

 opens at the posterior end through 

 the anus, which is not present 

 in the members of the ,phyla 

 already discussed. A muscular 

 pharynx (1) draws the fluids 

 into the long non-muscular in- 

 testine (2), through the walls of 

 which the nutriment is absorbed. 

 Just before the anal opening is 

 reached, the intestine gradually 

 becomes smaller; this portion is 

 known as the rectum. 



The excretory system consists of 

 two longitudinal canals (Fig. 111,7) 

 one in each lateral line ; these open 

 to the outside by a single pore (8) 

 situated near the anterior end 

 in the ventral body-wall (Fig. 

 112 a, c,P). 



A ring of nervous tissue surrounds 

 the pharynx and gives off two large 

 nerve-cords, one dorsal, the other 

 ventral, and a number of other 

 smaller strands and connections. 



