ROTIFER

also called wheel animalcule any of the approximately 2,000 species of microscopic invertebrates that constitute the class Rotifera, or Rotaria (phylum Aschelminthes). Rotifers are so named because the circular arrangement of moving cilia (tiny hairlike structures) at the front end resembles a rotating wheel. Although common in freshwater on all continents, some occur in salt water or brackish water; others live in damp moss or lichens. Most are free-living; some are parasitic. Some live as individuals; others live in colonies. Most rotifers are 0.1 to 0.5 mm (0.004 to 0.02 inch) long. The body may be spherical, flattened, bag-like, or wormlike. The body wall consists of a thin cuticle. Tufts of cilia at the anterior end make up the corona, which is used for feeding and locomotion. Small organisms are extracted as food from water currents created by the ciliated corona. Larger organisms, such as other rotifers, crustaceans, and algae, are also eaten. A mouth and digestive tract are usually present. The muscular pharynx, or mastax, contains hard, tiny jaws. Members of each species consist of an exact number of cells. In addition to the swimming rotifers, some (order Bdelloidea) loop along the bottom of ponds, alternately attaching the head and tail ends; others remain anchored by means of tubes or cases of jelly attached to the bottom. Modes of reproduction differ among species. Some species only reproduce parthenogenetically into females. Other species can reproduce both asexually and sexually. Males are smaller and less common in many species.