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Round worms. Slender, pseudocoelomate, unsegmented worms, circular in cross section. Some (eelworms) are serious parasites of plants, others are parasitic in animals and some are free-living in soil and marine muds.

Nematodes , or roundworms, are very important and found in nearly every ecological niche. There are at least 10,000 species, both free-living in aquatic and terrestrial habitats and as parasitic forms are very successful, found in virtually every type of animal. Nematodes can be carnivorous, herbivorous or parasitic.

The body is always long, slender and tapered at the end and circular in transverse section. A muscular pharynx is required to pump food through the gut since there is no peristalsis. Digestion is in two stages; initially extracellular and then phagocytosis occurs.

Most nematodes are dioecious, and the male nematode is usually smaller than the female. The posterior end of the male is curled like a hook to hold onto the female during copulation.


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The Phylum Nematoda is a member of the Superphylum Aschelminthes. Here is the complete "parentage" of Nematoda:

The Phylum Nematoda is further organized into finer groupings including:


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Adenophorea or Aphasmidia was a class of nematodes (roundworms). It has been by and large abandoned by modern taxonomy, because there is strong evidence for it being a motley paraphyletic group of unrelated lineages of roundworms. [more]


Secernentea are the main class of nematodes, characterised by numerous and an excretory system possessing lateral canals. Like all nematodes, they have no circulatory or respiratory system. [more]

At least 1,950 species and subspecies belong to the Class Secernentea.

More info about the Class Secernentea may be found here.


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Last Revised: August 22, 2014
2014/08/22 05:22:24