270 OTT. [Vol. VII. 



mother closes up and fits closely against the posterior wall of 

 the alimentary canal, thus leaving the posterior end of the body 

 bluntly rounded. This posterior body wall gradually grows 

 backward and becomes sharper until, in a quiescent worm, the 

 posterior end of the body is separated from the posterior end of 

 the alimentary canal by a space equal to the greatest width of 

 the body. 



That part of the alimentary canal posterior to the mouth is a 

 straight tube occupying the centre of the body as seen in cross- 

 section. In the living worm it varies in color from a light green 

 to a brown. This coloring is more marked in the intestine, 

 where the greater part of the color is due to the fact that the 

 food particles enclosed within the intestine can be seen through 

 the body tissues. 



There is but one opening to the alimentary canal, the mouth 

 opening (Fig. i, M). This opening is at the bottom of a cone- 

 shaped depression of the integument (Fig. 5, C.D^ which is 

 located on the ventral side of the body. When the animal is 

 at rest, the distance between this depression and the anterior 

 end of the body is about equal to the greatest width of the body. 

 This depression is very variable in its form. Sometimes it is 

 very long and slender, as in Fig. 5, and sometimes it is short 

 and thick. 



It is lined with ciliated epithelial cells similar to those of the 

 body wall, except that they are smaller. The muscular wall is 

 very thin and is made up of circular and longitudinal muscle 

 fibres. The circular fibres like those of the integument are next 

 to the epithelial cells. 



The alimentary canal has a distinct muscular wall by means 

 of which it moves independently of the movements of the body, 

 although this wall is not as contractile as the body wall. 



The alimentary canal is divided into two parts ; viz. pharynx 

 and intestine. 



Pharynx. — The pharynx, as seen in the living animal in a 

 quiescent state, is cask-shaped, being as long as the greatest 

 diameter of the body, and one-half as wide (Fig. i, /*//.). Its 

 anterior end opens through the mouth to the outside, and its 

 posterior end opens into the intestine. 



The wall of the pharynx is made up of an outer muscular 

 layer and an inner layer of ciliated cells. 



