Fig. 87.—Nectonema agile Verrill. A, The adult. Magnified. (After Fewkes.) B, Longitudinal section through the head. × about 20. (From Bürger.) a, Mouth; b, circumoesophageal commissure (dorsal); c, cell of salivary gland; d, septum cutting off head from rest of body; e, testis; f, ventral cord; g, oesophageal cells; h, lumen of oesophagus; i, cerebral ganglion (ventral).
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 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.
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 reproductive 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 Nematomorpha for their reception; and although Nectonema 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 of the Nematoda, and it must therefore be placed with it in the Nematomorpha.
On the other hand, it ought to be mentioned that Camerano[[210]] found that the chief details of the fertilisation and development of the egg in Gordius closely conform with what is known of the same processes in Nematodes, and he is of opinion that these resemblances are sufficiently important to justify the retention of the group among the Nematoda.
Life-History.—The life-history of Gordius comprises four stages—the early development of the egg, the first larval form, the second larval form, and the sexually mature form. Both larval forms are parasitic, and during their life they are actively engaged in feeding; the free form, on the other hand, takes in no nourishment, and is exclusively engaged in reproduction.
Fig. 88.—Abdomen of Pterostichus niger with the terga removed to expose the Gordius larva within. Slightly magnified. (From von Linstow.)
Von Linstow[[211]] gives the following account of the life-history of G. tolosanus, a form which has been more fully worked out than any other. In the month of April numerous specimens of the beetle Pterostichus niger were found floating on the surface of the ditches and small ponds in the fields surrounding Göttingen. Some were found dead or dying; others appeared quite healthy, and these were swimming actively, endeavouring to reach land. Within the abdomen of these beetles, in about 20 per cent of those collected, the second larval form of the G. tolosanus was found. The longest larvae were 122 mm. in length, and very soft, partly snow-white and partly brown in colour; traces of the boring apparatus of the first larval form were still to be seen, but in other respects the larva only differed from the free form in the immaturity of its sexual organs. Besides the parasite hardly anything was to be found in the abdomen of the beetle, the larva having eaten up all trace of the fat body and the generative organs of its host. The larvae bored their way out of the body of the beetle and became adult animals.