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 ?
Distribution of the turbellarian Urastoma cyprinae on the gills of the eastern oyster Crassostrea virginica.
2013), Urastoma-like turbellarian in gills of Ensis arcuatus (Lopez-Gomez et al.
The cell lineage of a polyclad turbellarian embryo reveals close similarity to coelomate spiralians.
No significant differences were observed in the number of turbellarians infecting the 2 gills (P = 0.9376) or the inner and outer demibranches of each gill (P = 0.3456).
Parasites Test 1 Organisms Gapesie M1 LNS Moribund Rickettsie 0 0 3.7 16.7 Gill ciliates 0 0 0 0 Gill Turbellarians 7.1 5.6 7.4 0 Intestine Turbellarians 0 0 3.7 0 Pearls 0 0 0 0 Parasites Test 2 Test 3 Organisms Gapesie MI LNS Gapesie Rickettsie 4.0 0 0 0 Gill ciliates 4.0 0 0 0 Gill Turbellarians 0 0 0 0 Intestine Turbellarians 0 0 0 0 Pearls 0 0 0 3.7
However, in the Beagle Channel mussel populations, a metacercaria parasitizing the foot and byssus gland was found, which was absent in the specimens here studied; conversely, turbellarians and copepods were absent in the Beagle Channel population (Cremonte et al., 2011), but were present in the specimens here studied.
Protists, nematodes, rotifers, small crustaceans, tardigrades, gastrotrichs, turbellarians, oligochaetes, and insect larvae are regularly found from psammon habitats (Thane-Fenchel, 1968; Whitman and Clark, 1984; Schmid-Araya, 1998, Kotwicki et al., 2005a, 2005b).
Other groups included turbellarians, polychaetes, oligochaetes, gastrotrichs, isopods, Kinorhyncha, and cnidarians.
Average densities of turbellarians, nematodes, physid gastropods, sphaeriid bivalves and elmid beeries were also relatively high (Table 3).
Free-living flatworms ("turbellarians" of the phylum Platyhelminthes) are common members of the taxonomically diverse meiofauna (Martens and Schockaert, 1986).