It consists of an internal cuticle, a well-developed subcuticula, and a circular muscular coat, the latter derived from the radial bands which originate from the body-walls, and act as divaricators. The anus is a more or less transverse slit, usually opening on the ventral aspect of the tail. It is, however, sometimes terminal or sub-terminal, e. g. Trichina, Trichocephalus, and the male Strongylidae, in which it is surrounded by a membranous umbrella-like expansion, or bursa. The anus is wanting in Ichthyonema and Mermis. Two or more unicellular glands are said to open on each side of it in some cases, e. g. in Dochmius.

An excretory system is probably always present, though it is not certainly known in all the small free-living species. The excretory pore is ventral and anterior near the head. A narrow canal, rarely ampullar as in Oxynris, runs backwards from it, and opens into a transverse vessel at about the level of the posterior end of the oesophagus. The transverse vessel is continuous on either side with a longitudinal vessel which runs backwards, imbedded in the corresponding lateral area at its internal margin. A vessel is also sometimes prolonged forwards on each side towards the head, and is then either continuous with the corresponding posterior vessel, or unites with a separate cross canal which opens however at the same external pore. There are few variations from this typical structure. The vessels end blindly; their walls consist of a granular substance with nuclei, and an internal highly refractile layer; their fluid contents are clear, and somewhat reddish, at least in the larger forms. Two unicellular glands open in many instances, one on each side of the excretory pore 1.

The Nematoda are of separate sexes with the exception of the genus Angiostomum, where one generation is composed of individuals which are, anatomically speaking, females, but in which spermatozoa are first of all formed, then ova, and the latter are thereupon impregnated. They are, therefore, physiologically hermaphrodite and self-impregnating. The genital organs are tubular. The testis is single; very rarely paired. It opens on the ventral aspect of the rectum close to the anus, and therefore the termination of the rectum is sometimes called cloaca. It stretches forwards on the dorsal aspect, or to one side of the alimentary canal, and may hook backwards anteriorly, or in the large Nematoda, even be disposed in several longitudinal coils. It is divisible according to peculiarities of structure, into the testis proper, which is terminal, vas deferens, vesicula seminalis, and ductus ejaculatorius. The walls of the whole tube are composed of a delicate membrane, lined in the testis and vas deferens by protoplasm containing nuclei, and disposed in lines which are longitudinal, and in the testis parallel, in the vas deferens interlaced, whereas the vesicula seminalis and ductus ejaculatorius have a distinct epithelium very variable in character in different genera and species.

In the vesicula each cell is in some of the larger species, e.g. A. megalocephala, provided with filamentous processes capable of changing their shape. The ductus ejaculatorius has a well-formed external layer of muscles, chiefly transverse. Properly speaking, the vesicula is only its upper and less muscular part. Two caeca of unknown function open into the male duct near its termination in Heterakis and Pelodera. The spermatozoa are peculiar. During their formation they are attached to a central, or in Trichina, Trichocephalus, and Trichosomum lateral, rhachis, which may become much divided, but they do not attain their characteristic shape until transferred to the female. In the male organ they are small truncate nucleated spheres. Four types of form and structure have been distinguished by E. van Beneden successively assumed in the uterus: (I) spheroidal - a layer of homogeneous substance covers one pole of the sphere; (2) pyriform - this substance assumes a conical shape; (3) campanuliform - the substance becomes a cornucopia-like figure, and is separated at its base from the protoplasm of the body of the spermatozoon by a limiting plate, from which a clearly defined line rises and traverses the cornucopia; (4) conoid - the cornucopia-figure is replaced by a cone, which consists of a superficial membrane, a layer of protoplasm, and an axial refractile rod.

One of the forms 2, 3, or 4, may effect impregnation. The protoplasmic body of the spermatozoon is capable of amoeboid motion 1. Copulatory organs take the form of spicules, and a 'bursa.' Spicules are wanting in Sphaerularia, Trichina, Dermatoxys. One is present in Oxyuris, Trichocephalus, and Trichosoma; two dissimilar in Heterakis, Filaria. etc.; two similar in all respects in the majority of genera 2. The spiculum is chitinoid, pointed, often dark-coloured, and either homogeneous or containing a soft core-It is contained in a sheath, which is an evagination of the dorsal wall of the cloacal region of the rectum, and with which its base is continuous. The sheath has a smooth internal surface except in most species of Trichocephalus, and in Trichosoma aerophilium, where it is covered with spinules. It is provided with retractor and exsertor muscles, and may, as in Trichocephalus, be partially extrusible. A bursa is not always present. In its least developed form it consists of a slight cuticular ridge on either side of the tail, which begins anteriorly to the anus. The ridges may become thin folds, which may or may not be connected in front of the anus, and sometimes stop short of the apex of the tail, or extend beyond it.

These membranes are best developed in Strongylus, where they form an umbrella-like expansion. The ventral area inclosed by the bursa is provided with sensory papillae, sometimes with ridges, spines, a sucker, as in the genus Heterakis, or five suckers (?) on either side the anus, as in Nematoxys ornatus1.

1Joseph states (Z. A. v. 1882, p. 605) that if the digestive tract of a living Ascaris megalocephala be injected with Carmine-albumen solution, and the worm be kept at the natural degree of warmth in a portion of a horse's small intestine, its body will be found on dissection to be traversed by a naturally injected system of fine branching canals, devoid of proper walls, and covering the surface of all the internal organs and muscle-cells. The coloured fluid passes by osmosis into the excretory vessels.

1 This motion has been actually observed, but not in Nematoda inhabiting warm-blooded Vertebrata. It is possible that in such cases the sperm is motile only at the natural temperature of the host. E. van Beneden draws attention to their great variability of outline in specimens preserved in Osmic acid.

2 The two spicules are occasionally fused at their apices, and their bases supported by an extra piece, e. g. in Angiostomum.