Nematodes are rather conservative in form, due in part to constraints imposed by their hydrostatic skeleton. The high internal pressure of their bodies affects the way they move, feed, and reproduce. Internal anatomy is often difficult to study on prepared slides because nematodes are difficult to stain and most internal structures are tube-like and coiled. This makes it difficult to distinguish among structures without dissection. Nematodes are studied best as temporary mounts in glycerine, which permits rolling the specimen to obtain the best view of particular structures.
Nematodes are typically elongate, tapered at both ends, and bilaterally symmetrical. They vary in size from microscopic to nearly 1 m long. They are pseudocoelomates, like acanthocephalans. However, unlike the acanthocephalans, nematodes are covered with a tough, relatively impermeable cuticle. The cuticle may have various longitudinal ridges, alae, or other ornamentations. Nematodes have a complete digestive system, with an anterior mouth and a posterior anus. The mouth of parasitic species is usually specialized and may be equipped with lips, sensory organs, and chitinous teeth or plates. A muscular esophagus connects with the buccal capsule and pumps food into the thin-walled, non-muscular intestine. Waste products are forcefully excreted from the anus. The nervous system comprises lateral nerve cords, and a prominent nerve ring surrounding the esophagus.
Most parasitic nematodes are dioecious. Males usually have a single set of reproductive organs. A tubular testis connects to a seminal vesicle, which in turn carries sperm to a vas deferens that terminates at a cloaca, where the anus is also located. The walls of the cloaca may be sclerotized to form supporting structures for 1 or 2 spicules, which open the vulva of the female and allow the amoeboid sperm to enter the uterus against the hydrostatic pressure of the female's body. The cuticle near the cloaca may be developed to form a copulatory bursa and/or papillae.
Females may have 1-6 sets of reproductive organs. Each tubular ovary is connected to an oviduct that in turn is connected to a uterus. Part of the oviduct may be enlarged to serve as a spermatheca. Eggs are expelled from the uterus through a vulva, that is located independently from the anus, and may be anywhere on the ventral surface of the body. A muscular ovijector may be present at the junction of uterus and vulva.
Some nematodes are viviparous and give birth to larvae, but most produce an egg. The shell of the egg is multilayered. It may be thin or thick, smooth-surfaced or rough. The zygote develops into a larva within the egg, and may emerge from the egg or await ingestion by its next host.
Nematodes pass through four larval stages prior to becoming adults. The larva usually has a similar structure to the adult, although they lack the cuticular ornamentation and the reproductive system is represented only by a few cells, the genital primordium. The larvae molt between stages and may retain the old cuticle for a while. These are termed ensheathed larvae. This old cuticle provides some protection against desiccation for larvae in the external environment.