138 



ORGANS AND ORGAN SYSTEMS 



the alimentary system, begins at the i8th somite and runs in a 

 straight course to the posterior end of the worm. It consists 

 of epithelial, vascular, circular and longitudinal muscular tissues 

 and is covered on the outside by peculiar yellowish-brown 

 chlorogogue cells derived from the coelomic endothelium. Along 



, 



FIG. 55. Stereogram showing the relations of organs in the posterior part of 

 the earthworm. (Worked out by Professors McGregor and Calkins, and drawn 

 by Miss Hedge.) 



the dorsal median line a longitudinal fold of tissue coming from 

 the dorsal wall of the intestine runs the entire length of this 

 organ as far as the rectum. This fold, called the typhlosole 

 (Fig. 55), has a different f-orm in different regions of the body 

 and contains additional blood vessels and chlorogogue cells, 



