168 BIOLOGY 



simple and crude, consisting only of the two layers of muscles, 

 longitudinal and circular, and the seta. The method of its 

 action is as follows: By the contraction of the circular muscles 

 the diameter of the body is reduced, and, inasmuch as the body 

 cavity is filled with the perivisceral liquid, and liquids are 

 incompressible, the contraction of the diameter of the body 

 must necessarily increase its length. The ends are thus pushed 

 apart; but the setae pointed backward act as anchors, and 

 the pushing of the two ends of the body apart will tend to push 

 the head forward, the rest of the body remaining practically 

 stationary. After the contraction of the circular muscles the 

 longitudinal muscles are contracted, thus shortening the length 

 of the body and at the same time increasing its diameter. 

 As the body shortens, the tail is pulled forward toward the 

 head, the setae again serving as anchors to prevent the body 

 from moving in the wrong direction. Thus by alternately con- 

 tracting the circular and longitudinal muscles, the head is 

 pushed forward and the tail is pulled up to the head. If the 

 earthworm wishes to move backward, it needs only to contract 

 the muscles connected with the setae and to point them for- 

 ward, when they will serve as anchors to prevent the body from 

 being pushed forward; and the alternate contraction of the 

 two layers of muscles will make the animal move backwards. 

 This alternate contraction of the muscles does not occur the 

 whole length of the body at once, but sections -may contract 

 or relax, causing waves of contraction to extend from one end 

 of the animal to the other. This method of locomotion is very 

 inefficient for an animal living on a flat surface, and the earth- 

 worm is only able to move slowly upon the ground. In his 

 underground burrows, however, where the animal nearly fills 

 up the burrow, the method of locomotion is much more efficient 

 and enables the animal to move with considerable rapidity. 



Alimentary System. As shown in Figure 83, the alimentary 

 canal consists of five layers. On the very inside next to the 

 cavity of the intestine is a layer of epithelial cells (Gr. epi = 



