VI 



PHYLUM NEMATHELMINTHES 



295 



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. 243, 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. 245, 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 

 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 de- 

 scription the pharynx 

 is also called stomo- 

 da3um. 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. 



Between the enteric canal and the body-wall is a distinct space, 

 the body-cavity, containing a clear fluid and more or less 

 encroached upon by the protoplasmic processes of the muscle-cells. 

 The cavity is bounded externally by these processes, internally by 

 the outer cuticle of the intestine : there is no trace of epithelial lining 

 such as occurs in most of the higher animals. The body-cavity of 

 the Nematode, in fact, does not exactly correspond to the coelome 

 to be met with in most higher phyla. It is not to be derived, 

 directly or indirectly, from the archenteron of the embryo, and 

 it does not lie. like a true ccelome, between layers of the 

 mesoderm. 



FIG. 244. Ascaris lumbricoides. A, a single muscle 

 fibre ; B, several fibres in transverse section with 

 portion of ectoderm (below), c, contractile substance ; 

 /. fibrous processes ; mi. nucleus ; p. protoplasmic 

 portion. (After Leuckart.) 



