Anatomy.—The Nematode worms, or thread-worms, form by far the largest and most important division of the group Nemathelminthes. The number of species is great, and although the conditions under which they live are of the most varied kind, there is, as a rule, little corresponding difference in structure, and hence the determination of the species is attended with no small difficulty.
With few exceptions the shape of the body is filiform (Figs. 66 and 71), the two ends being more or less pointed, and the posterior end of the male, which is generally a smaller animal than the female, is usually slightly recurved. The worms are, as a rule, white, or of the colour of polished ivory; they may be opaque or semi-transparent, but pigment spots are rarely developed.
Minute Nematodes abound in moist soil, around the roots of plants, etc., and may easily be detected with the aid of a lens wriggling about amongst the particles of sand and earth. Of the animal parasites perhaps the most familiar is the "round worm" (Ascaris lumbricoides, Figs. 66 and 67), which inhabits the alimentary canal of man; others are common in domesticated animals, as A. mystax in the cat and dog, and A. megalocephala in the horse and ox. They are also found living parasitically in plants (Fig. 77), causing the formation of galls and other pathological growths; Anguillula (Tylenchus) tritici causes in this way considerable damage to corn, and others attack root-crops, cabbages, etc. The "vinegar eel" (Anguillula aceti), which occurs so often in weak vinegar, is another familiar example of this group.
The Skin.—The body of the worm is encased in a relatively thick, transparent, smooth cuticle, which is turned in at the various apertures, and lines the tubes connected with them for a greater or less distance. The cuticle is in some cases raised to form spikes or hooks, and in certain species, e.g. Ascaris mystax and A. transfuga, it is produced into two lateral fins, which are supported by a thickened triradiate rod of specialised cuticle (Fig. 62); these fins, however, do not run far down the body. As a rule the cuticle is quite smooth, but it may be ringed, as in Filaria laticaudata and in F. denticulata; and the rings may bear backwardly-projecting teeth.
The skin of Nematodes consists of three layers—(i.) the above-mentioned cuticle, which is presumably secreted by (ii.) the sub-cuticle or epidermis which underlies it; the latter surrounds in its turn (iii.) the muscular layer.
The nature of the sub-cuticle is one of the debateable points in the morphology of the Nematoda. No cell outlines have been detected in it, although nuclei are scattered through it; it is in fact a syncytium, or protoplasmic mass in which cell limits cannot be distinguished. Many of the cells forming it have broken down into fibrils, and these form a close meshwork, which is occasionally specialised, as, for instance, round the nerve-cords. Along the median dorsal and ventral lines, and along the lateral lines, this tissue is heaped up in such a way as to divide the enclosed muscle-cells into four quadrants. These thickenings surround dorsally and ventrally a specialised nerve-cord, and laterally the excretory canals.
According to Jammes[[155]] this lack of differentiation in the sub-cuticular layer is caused by the early appearance of the cuticle, which he thinks is necessitated, at any rate in many of the parasitic forms, by the action which the digestive juices of the host would have on the otherwise unprotected body-wall.
Fig. 62.—A transverse section through the body of Ascaris transfuga Rud., in the region of the oesophagus: a, the muscular oesophagus with its triradiate lumen; b, the cuticle; c, the sub-cuticle; d, the muscular layer; e, the lateral nerves running in the lateral line; f, the excretory canal; g, the dorsal, and h, the ventral nerve; i, the triradiate rod in the fin.
The nervous system, according to the same writer, is of the same nature as this sub-cuticular tissue, only it is more differentiated, or perhaps we should say it has retained more of the primitive cellular character of the embryonic tissue. The fibres of the sub-cuticular tissue are closely connected with the fibrils which compose the spongioplasm (Fig. 64, d) of the muscles,[[156]] and form also the sheaths of the various nerves; in fact the passage of these fibrils into the nerves is so gradual that it is impossible to make any separation between them.