Roundworms are triploblastic protostomes. They are shaped like stereotypical worms, long and round in cross section, though without any segmentation. The body cavity is reduced to a narrow pseudocoelom, as is typical of very small animals. The mouth is anterior, and often surrounded by various flaps or projections used in feeding and sensation, with the anus slightly offset from the posterior. The epidermis secretes a layered cuticle that protects the body from drying out, from digestive juices, or from other harsh environments, as well as in some forms sporting projections that aid in locomotion.
Most free-living nematodes are microscopic, though a few parasitic forms can grow several metres in length. There are no circular muscles, so the body can only undulate from side to side. In order to actually get anywhere, the worm needs to be in contact with solid objects, its thrashing motions varying from mostly to completely ineffective at swimming. Different species feed on materials as varied as algae, fungi, small animals, fecal matter, dead organisms and living tissues.
Reproduction is usually sexual, typically with males slightly smaller than females and having a characteristically bent tail. In free-living species development is usually direct, with four molts of the cuticle occurring during growth. Parasitic forms often have quite complicated life cycles, moving between several different hosts or locations in the host's body. Infection occurs variously by eating uncooked meat with larvae in it, by entrance into unprotected cuts, by transfer via blood-sucking insects, and so forth.
Important parasites on humans include whipworms, hookworms, pinworms, ascarids, and filarids. Another roundworm of note is Caenorhabditis elegans, which lives in the soil and has found much use as a model organism.
The common presence of a pseudocoelom is not longer considered evidence that the pseudocoelomate phyla are all related, but a few groups are still probably close relatives of the Nematoda. Of special note here are the Nematomorpha, or horse-hair worms, which have larvae parasitic in arthropods and free-living adults. The Arthropods have also been considered to be possible relatives of these groups, the common process of ecdysis (molting) being evidence for this.