ALIMENTAEY CANAL OF VEEMES. 



159 



attains its liigliest development in the sporocyst forms of tlie Trema- 

 toda. Finally, tlie absence of an enteric canal is the rule among 

 the Cestoda, where the enteron is not present for a time even. 

 The enteron is altogether wanting in the Acanthocephali, and for 

 the same reason — namely, parasitism. 



Among the Platyhelminthes there are forms which are distin- 

 guished by the possession of an anus, and which may be contrasted 

 with those which indicate their lower condition by not possessing 

 one. Such are the Microstomete among the Tur- 

 bellaria rhabdocoela, and the Nemertina, the enteric 

 tube of which has pretty much the same form 

 throughout, and which begins by an elongated 

 ventral mouth, which lies behind the central nervous 

 system. In Malacobdella the mouth is placed at 

 the anterior end of the body. A muscular, but 

 generally feebly- developed, pharynx, leads into the 

 intestinal tube, which is provided with a large 

 number of lateral diverticula. This fills the greater 

 part of the body-cavity, to the walls of which it is 

 attached by muscular fibres. The lateral diverticula 

 of the enteric tube are sometimes regularly arranged, 

 and are the first indications of metamerism. This 

 is best seen in Pelagonemertes ; and so far this 

 form calls to mind the dendrocoelous Turbellaria. 





§ 129. 



In the Nemathelminthes all three portions of the 

 alimentary tube are generally present. In corre- 

 spondence with the form of the body, it forms a long- 

 tube, which traverses the body, beginning by a 

 mouth in the centre of the anterior end of the 

 body, and ending by a ventrally-placed anus, which 

 is more or less near the caudal end. The most 

 anterior portion (oesophagus) forms a narrow canal, 

 the walls of which pass gradually backwards into a 

 thick-walled pharynx (Fig. 70). This _ is distinctly 

 marked off from the rest, and is distinguished by 

 a musculature, which enables it to act as a sucking- 

 organ. The layer of chitin, which invests the tract 

 from the mouth to this portion, not unfrequently forms ridges or 

 tooth-like organs. The mid-gut (chyle-stomach), which succeeds 

 the pharynx, is, as a rule, the largest part; it has simple walls, 

 often formed by a single layer of cells, which in some (Heterakis 

 vesicularis, Oxyuris vermicularis) is provided in places with a 

 muscular covering of annular fibre. A cuticular layer generally 

 lies outside the epithehum ; an internal cuticle, which is traversed 

 by pore-canals, appears to be present also. In many, the mid-gut 

 forms a coecal diverticulum in its antei-ior portion. This portion 



Fig. 70. Alimen- 

 tary canal of a 

 Nematode (Dia- 

 gram) . 



