Turbellaria

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Related to Turbellarians: phylum Platyhelminthes, Polychaetes

Turbellaria

[‚tər·bə′lar·ē·ə]
(invertebrate zoology)
A class of the phylum Platyhelminthes having bodies that are elongate and flat to oval or circular in cross section.

Turbellaria

 

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.

REFERENCES

Beklemishev, 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

References in periodicals archive ?
Freshwater turbellarians from southern Brazil were investigated by Gamo and Leal-Zanchet (2004), Vara and Leal-Zanchet (2013) and Braccini and Leal-Zanchet (2013) in natural wetlands and agroecosystems.
solida was parasitized by prokaryotic inclusions in the digestive epithelium, intracellular inclusions of bacteria-like organisms and ciliates in the gills, gregarines similar to Nematopsis parasitizing exclusively the connective tissues of most organs, and a turbellarian similar to Paravortex in the intestine lumen.
Several planarians are known to affect bivalve mollusks, and among them turbellarians of the orders Rhabdocoela and Prolecithophora have been detected in the mantle cavity, gills, and alimentary tract of their host.
Distribution of the turbellarian Urastoma cyprinae on the gills of the eastern oyster Crassostrea virginica.
A Urastoma-Eke turbellarian was detected in gills of three Ensis macha sampled in May.
Dual origins of mesoderm in a basal member of the spiralian clade: cell lineage studies in the polyclad turbellarian Hoploplana inquilina.
From histological preparations, turbellarians can be observed adjacent to the gills or mantle, or within the lumen of gut and kidney (Bower 2004).
Turbellarians were then counted by separating the filaments with the aid of dissecting needles.
We determined almost the entire sequence of 18S rDNA in two species of dicyemid mesozoans and three species of turbellarians (Platyhelminthes).
Drills, crabs, and Stylochus turbellarians are all active predators in Delaware Bay (Stauber 1943, McDermott & Flower 1952, Maurer & Watling 1973, Ismail 1985) and likely do not leave boxes behind when they die or leave behind boxes of small individuals that rapidly disarticulate.
We noted the presence of three different parasites, rickettsies, gill turbellarians, and intestine turbellarians observed at low prevalence of 3.