Rotifera

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A phylum of pseudocoelomate, microscopic, mainly free-living aquatic animals, characterized by an anterior ciliary apparatus, the corona, whose cilia when in motion have the appearance of a pair of rapidly rotating wheels. This structure is implicit in the phyletic name (literally “wheel bearers”) and the older popular name wheel animalcules.

The Rotifera show considerable diversity in form and structure, but all are bilaterally symmetrical, pseudocoelomate animals possessing complete digestive, excretory, nervous, and reproductive systems, but lacking separate respiratory and circulatory systems. They possess two features unique to their phylum: the corona, which is a retractile trochal disk, and the mastax, which is a gizzardlike structure derived from the modified pharynx.

Rotifers are dioecious and sexually dimorphic; females are commoner than males, some of which are degenerate organisms lacking a mouth and digestive organs. Males, when produced in the life cycle, are short-lived and survive for only hours or at the most a few days.

The three major subdivisions of the Rotifera, now given class status, are the Seisonacea, Bdelloidea, and Monogononta. See also Bdelloidea; Monogononta; Seisonacea.

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