300 



ZOOLOGY 



SECT. 



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

 section, and it is lined by a cuticle secreted from the 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. 238, int.). 

 Posteriorly the intestine narrows considerably to form the short 

 rectum, which has a few muscular fibres in its walls and opens 

 externally by the anus (Fig. 240, an.). The food, consisting of the 

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

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



^ 



FIG. 239. Ascaris lumbricoides. A, a single muscle fibre ; B, several fibres in transverse 

 section with portion of ectoderm (below), c, contractile substance ; /. fibrous processes ; nu. 

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



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 stomodgeum. 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 

 endodermal, this portion of the canal being derived from the 

 archenteron of the embryo. 



