Defining Characteristics

The body of a nematode is long and narrow, resembling a tiny thread in many cases. Nematodes are commonly referred to as roundworms.

Systematics

Nematodes have a body cavity, but it is not a true coelom. As "pseudocoelomates," the body cavity lies between the mesoderm and the endoderm, rather than contained completely within the mesoderm. The epidermis of a nematode is highly unusual; it is not composed of cells like other animals, but instead is a mass of cellular material and nuclei without separate membranes. This epidermis secretes a thick outer cuticle which is both tough and flexible, maintaining the cylindrical shape of the roundworm. The cuticle is a feature shared with arthropods and other ecdysozoans. As in those other groups, the cuticle is periodically shed during the life of a nematode as it grows, usually four times before reaching the adult stage.

The head of a nematode has a few tiny sense organs, and a mouth opening into a muscular pharynx (throat) where food is pulled in and crushed. This leads into a long simple gut cavity lacking any muscles, and then to an anus near the tip of the body. Food digested in the gut is not distributed by any specialized vascular system, and neither is there a respiratory system for the uptake or distribution of oxygen. Rather, nutrients and waste are distributed in the body cavity, whose contents are regulated by an excretory canal along each side of the body.

Locomotion

The cuticle is the closest thing a roundworm has to a skeleton, and in fact the worm uses its cuticle as a support and leverage point for movement. Long muscles lie just underneath the epidermis. These muscles are all aligned longitudinally along the inside of the body, so the nematode can only bend its body from side to side, not crawl or lift itself. A free-swimming roundworm thus looks rather like it is thrashing about aimlessly.

The muscles are activated by two nerves that run the length of the nematode on both the dorsal (back) and ventral (belly) side. Unlike other animals, where the nerves branch out to the muscle cells, a nematode's muscle cells branch toward the nerves. The ventral nerve has a series of nerve centers along its length, and both nerves connect to a nerve ring and additional nerve centers located near the head.

Habitat and Feeding

Nematodes live not only in almost every geographic location on Earth, but live in such extreme habitats as ice and hot springs, as well as living on or in almost every other kind of animal and plant alive today. Free-living nematodes are extremely abundant in soils and sediments, where they feed on bacteria and detritus. Other nematodes are plant parasites and may cause disease in economically important crops. Still others parasitize animals (including humans); well-known parasitic nematodes include hookworms, pinworms, Guinea worm (genus Dracunculus), and intestinal roundworms (genus Ascaris).

Other Information

There are more than 15,000 known species of roundworms. If this estimated number of species is anywhere close to correct, it would mean that roundworms are the second most diverse group of animals, trailing behind only the arthropods.

Nematoda Links

http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html