ABSTRACT
Nematodes are fundamentally bisexual, but certain species are parthenogenetic, hermaphroditic or are pseudogamenous, in which development of the ovum is stimulated by a male gamete the nucleus of which does not fuse with that of the ovum and contributes nothing to the hereditary composition of the embryo. The nematode testis can be in the form of the widespread telogonic testis, in which the germ cells are produced from a single terminal cell, or the less common hologonic testis, in which proliferation of the germ cells takes place along the whole length of the testis. The male reproductive system generally comprises a testis (rarely two) that runs along most of the length of the nematode. The testis has a distal zone of germ-cell formation containing the spermatogonia, a zone containing the spermatocytes and spermatids, a seminal vesicle and a vas deferens that contains the sperm (see diagram in Gibbons, Chapter 2). Maturation of spermatids into spermatozoa appears to be triggered by mating and is thought to take place within the vas deferens (Scott, 1996). A muscularised region of the vas deferens forms an ejaculatory duct. The male copulatory organs generally include two innervated cuticular spicules (sometimes one), which are probably sensory and act as guide to sperm during copulation with the female. Some species posses a gubernaculum which guides the spicules during their eversion. A copulatory bursa is present in many species and aids in copulation by grasping the genital region of the female. The male copulatory apparatus is well endowed with sensillae. The female reproductive system comprises one or two ovaries, rarely more, a seminal receptacle, one or two uteri, a vagina vera and a vulva (see Chapter 2 on General Organization). There appear to be no sense organs associated with the female genitalia. Internal fertilization through female genital ducts is the general rule in nematodes. Hermaphrodites produce sperm that are used to fertilise the eggs of the same individual, as in Caenorhabditis elegans, but some hermaphroditic species occasionally produce a male which is capable of fertilising the hermaphroditic female, as in C. elegans. A few cases of traumatic insemination have been reported.
