278 



ZOOLOGY 



SEC'T. 



Digestive Organs. --The mouth leads into the anterior 

 division of the enteric canal, the pharynx or stomodoeu.m (Fig. 225 

 jiJt.) : its walls are very muscular, its cavity is three-rayed in cross- 

 section, and it is lined by a cuticle secreted from its epithelial layer 

 and continuous, at the mouth, with that of the body-wall. 

 Posteriorly the pharynx opens into the intestine (int.}, a thin 

 walled tube, flattened from above downwards, and formed of a 

 layer of epithelial cells bounded both internally and externally 

 by a delicate cuticle : it has no muscular layer (Fig. 223, int.). 

 Posteriorly the intestine narrows considerably to form the short 

 ri'rtmn, which has a few muscular fibres in its walls and opens 

 externally by the anus (cm.). The food, consisting of the semi- 



nu, 



m 



Fir;. 224. Ascaris lumbricoides. A, a single muscle fibre ; B, several fibres in transverse 

 section with portion of ectoderm (below) ; r. rnntractile substance ; /. fibrous processes ; n < ; . 

 nucleus ; p. protoplasmic portion. (After Leuckart.) 



fluid contents of the intestine of the host, is sucked in by move- 

 ments of the pharynx, and is then absorbed into the system 

 through the walls of the intestine. The food being already 

 digested by the host, there is no need of digestive gland-cells, 

 such as occur in animals which prepare their own food for 

 absorption. 



It will be noticed that in the above description the pharynx is 

 also called stomodseum. This must not be taken to indicate that 

 the two terms are synonymous, but that, in the present instance, the 

 epithelial lining of the pharynx is derived from the ectoderm, 

 being formed as an in-turned portion of the outer layer of the 

 body- wall. The epithelium of the intestine, on the other hand, is 



