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a class of flatworms (Platyhelminthes) that represents the most primitive group of bilaterally symmetrical animals. The ciliated body, which ranges in length from fractions of a millimeter to 35 cm, is spindle-, droplet-, leaf-, or ribbon-shaped. Small turbellarians move by means of cilia, and large turbellarians by means of muscle contractions. There is no body cavity, and the spaces between internal organs are filled with parenchyma.
The mouth is located on the abdominal side of the body, either near the middle or at the anterior or posterior end, and usually extends into the muscular pharynx. In acoels, a primitive form of turbellarians, digestion occurs in special digestive cells or in parenchymatous cavities. Other turbellarians have a pouchlike or branched intestine without an anal opening. There are no organs of blood circulation. Respiration is cutaneous. Protonephridia, the organs of excretion, are absent in primitive turbellarians. The nervous system in lower turbellarians is diffuse and embedded in the cutaneous epithelium; in more highly organized turbellarians it consists of cephalic nerve ganglia and form one to six pairs of longitudinal trunks joined by transverse septa. The sense organs include eyes, olfactory pits, and tactile hairs and tentacles; sometimes there is a statocyst—an organ of equilibrium.
Turbellarians are hermaphrodites. Frequently, part of the ovary is converted into a vitellarium, which supplies the embryo with nourishment in the form of yolk cells. The majority of turbellarians are characterized by direct development, although in some polyclads the embryo develops into a Müller’s larva. In addition to sexual reproduction, some turbellarians are characterized by asexual reproduction through transverse division.
The class Turbellaria includes 11 or 12 orders, embracing approximately 3,000 species. The flatworms are distributed in seas and fresh waters at all latitudes; terrestrial planarians inhabit tropical rain forests. Most turbellarians are predators; a few marine forms parasitize echinoderms, mollusks, and other animals. According to E. Metchnikoffs theory of phagocytella, turbellarians are directly descended from the phagocytella-like ancestors of multicellular animals. The most primitive turbellarians are acoels and the group Xenoturbellida; the remaining turbellarians descended from forms closely related to them.
REFERENCESBeklemishev, V. N. “Klass resnichnykh chervei (Turbellaria).” In Rukovodstvopo zoologii, vol. 1. Moscow-Leningrad, 1937.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vols. 1–2. Moscow, 1964.
Zhizn’ zhivotnykh, vol. 1. Moscow, 1968.
Dogel’, V. A. Zoologiia bespozvonochnykh, 6th ed. Moscow, 1974.
Ivanov, A. V., and Iu. V. Mamkaev. Resnichnye chervi (Turbellaria), ikh proiskhozhdenie i evoliutsiia. Leningrad, 1973.
A. V. IVANOV