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Roundworms have a ganglial nervous system with an anterior neural ring representing (evolutionarily) a primitive cephalization.

Nematodes have two main longitudinal ganglial chords that extend one dorsally and the other ventrally under the epidermis. There may also be nerves lateral to these main chords. The nervous system of a free-living nematode, “Caenorhabditis elegans”, has been well-studied in neurophysiological research and presents 302 neurons.

The nematode “C. Elegans” was the organism used in the research on the genetic regulation of organogenesis and apoptosis whose researchers won the Nobel prize of Medicine in 2002 (Brenner, Horvitz and Sulston).

The metabolic residuals of nematodes are collected by two longitudinal lateral excretory channels that open in one single excretory pore near the mouth.

The cylindrical shape of nematodes made impossible the respiration exclusively by simple diffusion among cells since there are tissues far from the exterior. This problem was solved by the presence of an inner cavity in the body filled with fluid, the pseudocoelom. The pseudocoelom has the function of distributing gases and nutrients to the body and to collect residuals, besides serving as a hydrostatic base to keep the worm shape.

(For the fact that the pseudocoelom fluid and the pseudocoelom do not characterize a true circulatory system with blood and heart it is not said that in nematodes the respiration is cutaneous; it is considered that these animals still make respiration by diffusion).

Nematodes, like annelids, have a cylindrical elongated body. Annelids differentiate from nematodes by presenting a segmented body (body divided into metameres) and so they are called segmented worms.

Nematodes are also known as roundworms. As the name indicates they are not flat like platyhelminthes. In evolutionary grounds with the nematodes the first complete digestive system appears, with mouth and anus, and the pseudocoelom is also a novelty.

The main evolutionary innovation of nematodes is the complete digestive system, with two openings (mouth and anus).

Since the ingestion and the defecation processes can occur in different extremities of the digestive tube, beings with a complete digestive system have the advantage of ingesting new food while residuals of already eaten food are still inside the body and not yet eliminated.

Just like platyhelminthes, nematodes are triploblastics, i.e., they present three germ layers (ectoderm, mesoderm and endoderm).

There are parasitic roundworms, including parasites of plants, but there are also free-living nematodes.

Ascaris, hookworm and filaria, all parasites of humans, are examples of nematodes (also known as roundworms).

The main human diseases caused by nematodes are ascariasis, ancylostomiasis (hookworm infection) and filariasis (commonly known by its consequence, elephantiasis).