THE REPRODUCTIVE ORGANS.—With the exception of the genera _Angiostomum_, _Pelodytes_, and of _Rhabdonema nigrovenosum_, which are physiologically hermaphrodite and self-impregnating, the Nematodes have separate sexes. The males are, as a rule, smaller than the females, and may usually be distinguished by the posterior end of the body being curved towards the ventral surface; a genital bursa, and one or more spicules are often found in this sex. Further, the position of the genital opening differs; in the male the vas deferens opens on the ventral surface of the rectum close to the anus, but the oviduct in the female opens in the ventral middle line, usually near the middle of the body, but sometimes close behind the excretory pore, or in some Strongylidae just in front of the anus. The tail of the male bears very numerous papillae, which are of considerable systematic importance. [Illustration: FIG. 66.—_Ascaris lumbricoides_ Cloq. ♂, natural size, cut open along the dorsal middle line. _a_, Oesophagus; _b_, intestine; _c_, testis; _d_, vas deferens; _h_, lateral excretory canals.] With rare exceptions, _e.g._ _Filaria attenuata_, where it is double, the male reproductive organ consists of a single tube divisible into a testis proper, a vas deferens, a vesicula seminalis, where the spermatozoa are stored up, and a ductus ejaculatorius. The tube stretches through the body in a straight line in the small free-living forms, but is thrown into loops and coils in the larger parasitic Nematodes. Within the testis the mother-cells of the spermatozoa are attached to a rhachis or axial cord; the mother-cells divide, and their products ultimately form spermatozoa. The latter have a very peculiar shape; in accordance with the universal absence of cilia in the Nematoda the spermatozoon has no flagellum, and at first consists of a spherical nucleated cell, on one side of which a cap or covering of some refractive substance appears. The cap elongates and {135}becomes conical, whilst the protoplasmic portion of the spermatozoon throws out pseudopodia and becomes amoeboid, but ultimately rounds itself off again. The spermatozoa do not attain maturity until they reach the uterus of the female. The internal female reproductive organs are, with few exceptions (_Trichina_, etc.), double, but the vagina, which is lined with cuticle continuous with that covering the body, is always single. They are usually much coiled, and may be divided into ovary, oviduct, and uterus. The ova arise from a polynucleated mass of protoplasm or syncytium (Fig. 65, _o_) at the upper end, and acquire distinctness as they approach the oviduct. Fertilisation takes place in the uterus, but the segmentation may not begin until some time after the eggs are laid; in _Dochmius_, however, it is well advanced at this period, and in many genera, _e.g._ _Pseudalius_, _Trichina_, _Dracunculus_, etc., the whole development of the larva takes place in the body of the mother.