THE EARTHWORM AND OTHER SEGMENTED WORMS 169 



cross section of an animal (Fig. 97) will reveal a thick body wall 

 made up principally of two layers of muscles, a layer of circular 

 muscles (circ. mus) running around the body just beneath the 

 skin, and a layer of longitudinal muscles (long, mus) underneath 

 the circular ones. It is evident that when the circular muscles 

 near the anterior end contract, the body becomes thinner and 



bw 



FIG. 96. Diagram of the internal anatomy of the earthworm. 



bw, body wall ; dv, dorsal vessel ; i, intestine ; iw, intestinal wall ; Inv, 

 lateral neural vessel ; n, nephridium ; nc, nerve cord ; pv, parietal vessels ; s, 

 septa ; snv, sub-neural vessel ; t, typhlosole ; w, ventral vessel. (After Jammes.) 



therefore extends, and when they relax and the longitudinal 

 muscles contract, the rest of the animal is drawn forward by the 

 shortening caused thereby. The division of the body into 

 rings or segments aids in the activities of these muscular layers. 

 USE OF BRISTLES IN LOCOMOTION. The question naturally 

 arises as to what keeps the anterior end of the body from being 

 drawn back by the contraction of the longitudinal muscle. A 

 similar question, why is it so difficult to drag an earthworm 



