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 {126}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. [Illustration: 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.