Invertebrate Zoology: A Study Guide - T. A. Dauda 2014
Metazoa
Roundworms, or Nemathelminthes
Over 10,000 species of Roundworms are known to science. These worms inhabit A wide variety of environments, including marine and freshwater ecosystems, soils, and decomposing organic matter. Many species have adapted to a parasitic lifestyle in plants, animals, and humans. Free-living roundworms are typically small organisms, measuring up to 1 mm in length (with only a few species reaching up to 50 mm). Parasitic forms can also be microscopic, yet certain individuals grow up to 20–40 cm long.
All members of this phylum share several common features. The body is unsegmented, possessing a primary body cavity (pseudocoelom). The Mouth is located at the anterior end of the body, and the Digestive System consists of foregut, midgut, and hindgut regions, terminating in an anus. The Muscular System is partially reduced, featuring only longitudinal Muscles arranged in four distinct bands. Circulatory and respiratory systems are absent. The vast majority of species are dioecious (sexes separate).
Among the several classes comprising the phylum of roundworms (Nemathelminthes), the Class Nematoda, or true roundworms, holds the greatest practical significance and species diversity.
Last update: 13/08/2026
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