[Illustration: FIG. 64.—A, transverse section through the centre of a muscle-cell; B, the same through a nerve fibre showing the sub-cuticular fibres running into the sheath. (After Rohde.) _a_, Cuticle; _b_, sub-cuticular fibres continuous with _d_; _c_, contractile columns; _d_, network of spongioplasm; _e_, nucleus.] In addition to the characteristic muscles of the body-wall there are others, such as those which move the spicules in the male, which cross the body-cavity obliquely near the anus, and such as sphincter muscles near the latter orifice, which have not {130}the characteristic arrangement of contractile and medullary parts described above. THE BODY-CAVITY.—The skin of a Nematode, as described above, contains most of the important organs of the body within its thickness. The chief muscular system, the nervous system with its sense organs, and the excretory organs are all embedded in or form part of the skin, which in its turn encloses a cavity—the body-cavity—in which the other two systems of organs which are found in Nematodes lie. These are the digestive system and the reproductive system. The body-cavity is continuous from one end of the animal to the other, and is in no case divided up into compartments by the presence of septa or mesenteries. It contains a coagulable fluid with numerous corpuscles; this is, as a rule, colourless, but in _Syngamus trachealis_ Sieb. (Fig. 70), which lives on blood, the haemoglobin of its host tinges it red, though the colour is said to disappear if the parasite be isolated and starved. The morphological nature of this body-cavity affords an interesting problem. It is not a true _coelom_, such as exists in the earthworm, since it is not surrounded by mesoderm, nor do the excretory organs, with the possible exception of one or two genera, open into it, nor do the generative cells arise from its walls. Essentially it is a space between the mesodermic muscle-cells which line the skin and the endodermic cells of the alimentary canal, and although in many of its functions it resembles the coelom of other animals, its morphological character is quite different. There are no respiratory or circulatory organs in the Nematoda; possibly the fluid in the body-cavity acts, to some extent, as a carrier of oxygen, but from the inert and almost vegetative life of these animals it seems probable that their respiratory processes are slow, and in fact Bunge[161] has shown that _Ascaris mystax_, found in the intestine of the cat, will live for four or five days in media quite free from oxygen, and that _A. acus_ from the pike will live and exhibit movements in the same media for from four to six days.