Nematodes are one of the most widely distributed and best adapted phylum of organisms on the planet. These worms, ranging in size from microscopic to over a meter in length, can be found essentially anywhere on the planet, from the peaks of mountains to the bottom of the ocean. Currently, over 28,000 species have been described, 16,000 of which are parasitic. There are nematode parasites of plants, insects, fish, birds, and mammals.
They are typically small worms, under 3 mm long, although some species are much larger and have been known to reach over a meter. They have a distinct, radially symmetrical head with sensory bristle and a mouth. The mouth can have varying numbers of lips, which often bear teeth. Nematodes do not have stomachs, just an intestine connected to the mouth and rectum. They have four nerve cords which run the length of the body, and connect near the head to form a structure to serve as the brain. Their bodies are covered with sensory bristles to provide a sense of touch.