78 EARTHWORMS, LEECHES, AND SEAWORMS 



The notable thing about the body walls is the two layers 

 of muscular tissue each composed of muscles running in 

 opposite directions. The wrigghng, crawling movements 

 of this animal are produced by these two layers of muscles. 



The bristles, or setae of the earthworm. — The body of 

 the earthworm is furnished with four double rows of stiff, 

 chitinous bristles, or setae. They may be felt by drawing 

 the worm backward across the hand. There is one double 

 row along each edge of the ventral surface and one double 

 row along the lower part of each, side of the body (Fig. 40). 

 Each segment, except the first two or three and last, 

 has four pairs of setae and each pair is provided with mus- 

 cles so that the bristles may be turned and held in various 

 directions, and extended or withdrawn. When a bristle 

 wears out, it is cast off and a new one grows in its place. 



Movement of the earthworm. — If an earthworm is placed 

 on a piece of glass or other smooth surface, it will squirm 

 and wiggle but will make no progress. It is capable of 

 motion on a smooth surface but cannot change its location. 

 This is because the setae, which perform an important func- 

 tion in the locomotion of the animal, are unable to do their 

 work on smooth surfaces. Under ordinary conditions the 

 bristles stick into the soil and prevent the worm from sUp- 

 ping backward, when the muscles contract to force the body 

 forward. On hard smooth surfaces the bristles are unable 

 to get a hold and the worm, despite its struggles, re- 

 mains in one place. When the worm desires to move for- 

 ward, it points the setae backward and they stick into the 

 soil. The longitudinal muscles, which are then contracted, 

 pull the posterior end of the body forward and shorten and 

 thicken the whole body. The circular muscles now con- 

 tract, thus forcing the body to become thinner and longer. 



