The name sometimes given to a phylum of worm-like animals that are typified by possessing a pseudocoelom. True segmentation is not present. The intestine is straight with a highly developed pharynx and anus. Cell constancy is characteristic of the group: after the young has hatched (or been born) the number of cells in the body never increases although the size of the body usually does. There are six classes, some free-living and some parasites. The group contains several miscellaneous groups whose classification has always been controversial; the classes are now generally thought to be unrelated and are usually regarded as distinct phyla. See Gastrotricha; Kinorhynchia; Nematoda; Nematomorpha; Priapulida; Rotifera.
Subjects: Zoology and Animal Sciences.