224 ZOOLOGY. 



further differentiation, the remainder of the tube being fairly 

 uniform and called the intestine. Frequently however special 

 enlargements occur, chief among which is the stomach. In 

 the leeches the alimentary system is much modified in accord- 

 ance with the blood-sucking habit of the animal. The crop 

 is capable of great enlargement and may contain enough 

 blood to nourish the animal for a long time. The mouth is 

 sometimes armed with special cuticular outgrowths which 

 serve as teeth. Glands either unicellular or compound occur 

 in various regions of the digestive tract. In the earthworm 



FIG. 102. 



ent. 



FIG. 102. Transverse section of the intestine of the Earthworm, ty, typhlosole, an 

 infolded longitudinal ridge in the gut in which the gland cells (.gl.) are especially 

 aggregated. Other letters as in Fig. 101. 



Questions on the figure. Of what conceivable gain is the typhlosole? 

 What is it analogous to in the higher types of animals? 



and related forms there is a dorsal longitudinal fold of the 

 intestinal wall into the lumen of the tube, thus increasing the 

 exposed surface. This is called the typhlosole (Fig. 102) and 

 is supplied with cells which have been described as digestive. 

 The entodermal epithelium may secrete a cuticle or may be 

 ciliated. This layer is surrounded by a connective tissue and 

 muscular fibres. 



262. Respiration is effected for the most part through the 

 general body wall, into which the blood capillaries or the 



