; phylum Aschelminthes)
A class (or phylum in some classifications) of acoelomate, unsegmented animals in which normally a complete alimentary canal is present, as is a muscular pharynx possessing well-developed jaws. Their name is derived from the ciliated crown which in many species gives the appearance of a rotating wheel when it beats. The largest individuals reach 3 mm in length, but most are much smaller. Most are solitary and free-moving, but some are sessile and some colonial. They occur mainly in freshwater habitats. They swim by means of their cilia, or crawl across a substrate by muscular movements. Most are benthic. The body is always covered by a cuticle, which may be ornamented. There is a pseudocoelom between the body wall and gut. Some feed on suspended matter, others are predatory on Protozoa, rotifers, or small metazoan animals. Most are non-parasitic. All (except Bdelloida) reproduce sexually. There are about 1800 species, grouped in two classes: Digononta, comprising the orders Seisonidea and Bdelloida, and containing rotifers with two ovaries; and Monogonta, with one ovary, comprising the orders Flosculariacea, Collothecacea, and Ploima.
Subjects: Zoology and Animal Sciences — Ecology and Conservation.