78 liARTHWORMS, LEECHES, AND 8EAW0RMS 



The notable thing about the botly walls is tlie two layers 

 of muscular tissue each composed of muscles running in 

 opposite directions. The wriggling, crawling movements 

 of this animal are pr(i(hjced 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 setie. 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 doulale 

 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 setaj and each pair is provided with mus- 

 cles so that the bristles may l:e turned and held in various 

 directions, and extended or withdrawn. When a Ijristle 

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



Movement of the earthworm. — If an earthwoi'm is placed 

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

 and wiggle but will make no progress. It is capal^le of 

 motion on a smooth surface but cannot change its location. 

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

 tion in the locomotion of the animal, ai-e mialile to do their 

 work on smooth surfaces. Under ordinary conditions tlie 

 bristles stick into the soil and ]irevent the worm from slip- 

 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 sti'uggles, re- 

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

 ward, it points (:he setoe backward and they stick into the 

 soil. Tlie longitudinal muscles, which are then contracted, 

 l)ull tlie posterior end of the body foi'ward and shorten and 

 tliicken the whole body. The circular nmscles now con- 

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



