EARTHWORMS, LEECHES, ANB SEAWOBMS 



79 



But since the setae will not allow the body to slip backward, 

 it is forced in a forward direction. By repeating these 

 operations the worm 

 progresses slowly from 

 place to place. Very 

 often the earthworm 

 desires to travel back- 

 ward, especially when 

 it wishes to back 

 quickly into its bur- 

 row. In this case the 

 setse are pointed for- 

 ward and the same 

 muscular contractions 

 suffice for the back- 

 ward movement. 



The digestive sys- 

 tem. — The alimenta- 

 ry canal runs straight 

 through the body 

 from end to end. It 

 consists of several 

 fairly distinct parts, 

 each with its own pe- 

 culiar function. The 

 mouth is a simple, 

 crescent-shaped open- 

 ing at the anterior 

 end of the body, and 

 is overhung by the lip, or prostomiuin. This worm has no 

 teeth nor tongue. Beginning at the mouth is the barrel- 

 shaped pharynx, extending through several segments. 



Fig. 41. — Earthworm : 1, anterior end 

 opened along dorsal side ; 6, brain ; e, gullet ; 

 t, seminal vesicles; u, crop; g, gizzard; 

 bv, blood vessel; i, intestine; 2, anterior 

 end ; m, mouth ; s, setse. 



