vi NEMATOMORPHA CLASSIFICATION 169 



chamber by a transverse septum placed a little way behind the 

 head. The anterior chamber contains the brain and is lined by 

 a definite epithelium, the posterior is not. The layers of the skin 

 correspond with those of Nematodes or of Gordius, but the hypo- 

 dermal cells show no cell outlines ; still they are not so modified as 

 in the former grcyip. The hypodermis is thickened in the median 

 dorsal and ventral line, and the single nerve-cord lies in the latter. 



The alimentary canal is degenerate, as in Gordius. A mouth 

 exists, but it is minute, and opens into a very fine tube lined 

 with chitin, which pierces through the substance of a single 

 elongated cell. This minute oesophagus, with its coextensive cell, 

 reaches back to the transverse partition, but behind this a few 

 other cells become associated with it, and ultimately the lumen 

 of the alimentary canal is surrounded by four cells ; but the 

 number diminishes behind, and soon only two cells surround the 

 tube at any one level, and the intestine dwindles away some little 

 distance in front of the tail. There is no sign of an anus. A 

 circumoesophageal nerve-ring exists, of which the ventral part is 

 by far the larger (Fig. 87); it gives off a ventral nerve-cord, 

 which swells posteriorly in the male into a large anal ganglion, far 

 bigger than the brain, and larger in the male than in the female. 



The testes consist of a dorsally placed sac, continuous behind 

 with a vas deferens ; this opens at the posterior end, which is 

 pointed and slightly curved ventrally. The ovary is unknown ; 

 but females have been found with their body-cavity crammed 

 with ova ; these escape, like the spermatozoa, from a genital pore 

 at the posterior end of the body. 



Classification. The separation of the Nematomorpha from the 

 Nematoda depends mainly on the character of the nervous system, 

 the absence of the lateral lines and of the dorsal line, the character 

 of the contents of the body-cavity, and the character of the reproduc- 

 tive organs. In Gordiidae the latter are always placed dorsal to 

 the intestine, and ovaries and testes open alike at the hinder end of 

 the body. The importance of the differences in the organs just 

 enumerated has been considered sufficient to justify the removal of 

 the Gordiidae from the Nematoda, and the establishment of the 

 special sub-Order Nematomoqma for their reception ; and although 

 Xcctonema has a dorsal line, and is in some other respects 

 intermediate between the two groups, there can be little doubt 

 that it is more closely allied to Gordius than to any member 



