The only other genus which is associated with _Gordius_ in the group Nematomorpha is _Nectonema_, of which there is as yet but one species known, _Nectonema agile_ Verr.[207] Our knowledge of the anatomy of this worm is due mainly to Bürger[208] and Ward.[209] _Nectonema_ is a marine worm found swimming near the surface of the sea with rapid undulatory motion. The males are from 50 to 200 mm. long, the females from 30 to 60 mm. The body is faintly ringed, and bears two rows of fine bristles on each side. Owing to a curious torsion of the body through a right angle, the lateral bristles of the anterior third seem to be placed in the ventral and dorsal middle line. They are very easily broken off. The body is divided into a small anterior and a large posterior {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 hypodermal cells show no cell outlines; still they are not so modified as in the former group. 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.