a class of worms of the subphylum Nemathelminthes, or Aschelminthes. Some Rotatoria form colonies that measure 3–4 mm in diameter. The body, which measures 0.03–2.5 mm, is elongated or, more rarely, spherical. It is usually divided into a head, trunk, and foot (which is sometimes completely reduced). On the head is a rotatory apparatus (hence the name) which in the simplest form consists of two ciliated bands and serves for locomotion and the procuring of food (the movement of the cilia creates a whirlpool that draws small food particles into the mouth). Many Rotatoria have a shell. The muscular, digestive, excretory, nervous, and genital systems of the body are located in the primary cavity. The males are dwarfs, usually severely reduced, and not found in all species. Some Rotatoria combine unisexual and bisexual reproduction (heterogony); others are exclusively unisexual (parthenogenesis). There are approximately 2,000 known species; approximately 700 species are found in the USSR. Rotatoria live wherever there is water. They play a significant role in the self-purification of bodies of water. Large groups of these worms are sometimes found in plankton and serve as food for other animals.