Most nematode species have males and females, although, depending on the mode of reproduction, in some cases males are more rare than females/hermaphrodites. The reproductive tract of the female/hermaphrodite is essentially a tube filled with maturing eggs. Depending on the species there may be one tube (a single gonad) (Fig. 5A, B) or there may be two tubes (a pair of gonads); these systems are respectively called monodelphic and didelphic. The reproductive tracts may be outstretched (Fig. 1A), reflexed (Fig. 5B, C) or coiled (Fig. 5D). Tracts that are prodelphic are anteriorly directed (Figs. 5 A,B, D), whereas didephic systems may be amphidelphic, having tracts in opposing directions. Often monodelphic systems have a postvulvar sac, a small chamber extending opposite to the gonad and posterior to the vulva opening (Fig. 5A).

Male nematodes also may have one gonad, called monorchic and this may be reflexed anteriorly (Fig. 6A, B). A diorchic male system has two gonads, typically oriented in opposing directions, but joining at a shared duct (Fig. 6C).

Upon closer examination, the female/hermaphrodite and male reproductive tracts, enclosed by an epithelial sheath, include highly specialized regions (Fig. 7). In most nematodes the developing germ cells progressively mature from the distal end marked by a cap cell, thus points of maturation are distinguished as germinal zone and growth zone in both the ovary and testis (Fig. 7). In females/hermaphrodites the ovary leads to an oviduct that may include a structure specialized for sperm storage, the spermatheca (Fig. 7A). The spermatheca leads to a muscular uterus, that is functionally involved in expelling eggs through the vagina to the vulva opening (Fig. 7A). The testis may terminate at a seminal vesicle specialized for storing sperm, and sometimes set off from the growth zone by a constricted region, the vas efferens (Fig. 7B). Proximally a vas deferens leads to a common duct with the digestive tract, the cloaca, thorugh which sperm are released. The male reproductive system is associated with complex accessory structures for mating (Fig. 8). These include a pair of cuticularized spicules which are protrusible by a complex set of muscles (Fig. 8A). Together the spicules form a conduit, visible in cross section, thorugh which sperm pass (Fig. 8B). The spicules move in a channel that posteriorly may have a heavy cuticular lining, the gubernaculum (Fig. 8). Both the male and female/hermaphrodite reproductive tracts may include a range of specialized glands (Fig. 7).