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 



F IG - 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 wa^ll 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 form in different regions of the body 

 and contains additional blood vessels and chlorogogue cells, 



