Phylum Nematoda

Common Name: Roundworms

Etymology: "Thread-forms", from the Greek nema for thread and eidos for form.

Representative examples:

Click images to enlarge.
Free-living nematode.	Yellow nematode.	Monocus, a predatory soil dweller	SEM of a free-living interstitial nematode from marine sand	A living nematode worm viewed among the cyanobacteria it eats.

Additional Information:

A very speciose group (more than 25,000 species are known) of pseudocoelomate protostome invertebrates, the nematodes are essentially ubiquitous, occuring in every major habitat and parasitizing a wide variety of plants and animals.

"One species is known that can live in old vinegar (Turbatrix aceti)and another that as only been found in German beer mats. Though only about 80 000 species have been described some scientists estimate there may be as many as a million species all told. They can occur in very dense numbers in the soil and rotting vegetation, as many as 90 000 have been found in a single rotting apple, while millions occur in the top 3cm (1 inch) of a square metre of good quality soil. While there are a huge number of free living Nematodes there are also a large number of parasitic species, many of which cause diseases to man and other animals as well as to plants, nearly every living organism has been found to be parasitised by one species of nematode or another. Most nematodes are reasonably small, they range in size from 100 micrometres in length (1/10th of a mm or 1/250th of an inch) to the female Giant Nematode Dioctophyme renale which may be up to 1 metre, or 3 ft long."
- from Phylum Nematoda, 2004, © Earthlife.net.

The nematodes are distinguished from flatworms in that they possess a simple, but complete digestive tract and a pseudocoelom (body cavity bound by mesoderm on one side only). They are generally small, unsegmented, and have a thick, protective outer cuticle which allows them to occupy harsh environments like the digestive tracts of hosts and acts as a support structure for their bodies. Nematodes have a simple nervous system with a cerebral ganglion in the form of a nerve ring around the mouth. A "wreath" of sensory and motor nerves hang down from this ring and ventral and dorsal nerve trunks extend the length of the body from the ring ganglion. Roundworms also possess unique excretory structures called renette cells, which are not homologous to the protonephridia of any other metazoan. Because nematodes have only longitudinal muscles, not circular one, their locomotion is limited to whip-like undulations, relying on contact with a substrate, against which the body pushes. As a group they lack circulatory and gas exchange structures, accomplishing the latter functions via diffusion and movement of fluids in the relatively small body cavity.

Information in this section adapted from Brusca and Brusca, 2003, The Invertebrates, 2nd edition, pp. 351-362.

Link:

• Nematodes (Earth Life Web)