planarian


planarian
/pleuh nair"ee euhn/, n. Zool.
any of various free-swimming, mostly freshwater flatworms of the class Turbellaria, having an undulating or sluglike motion: popular in laboratory studies for the ability to regenerate lost parts.
[1885-60; < NL Planari(a) a flatworm genus (n. use of fem. of LL planarius level, on level ground; taken to mean "flat"; see PLANE1, -ARY + -AN]

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Any of about 3,000 species of widely distributed, mostly free-living flatworms of the family Planariidae and related families (class Turbellaria), usually found in freshwater but also in marine and terrestrial environments.

The soft, ciliated body is leaf-shaped when elongated. The spade-shaped head has two eyes and sometimes tentacles. The tail is pointed. The mouth is on the lower side, often more than halfway toward the tail. Most species are 0.1–0.6 in. (3–15 mm) long; some grow to about 1 ft (30 cm). Planarians swim with an undulating motion or creep like slugs. Most feed at night on protozoans, snails, and other worms.

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      any of a group of widely distributed, mostly free-living flatworms of the class Turbellaria (phylum Platyhelminthes). Planaria is the name of one genus, but the name planarian is used to designate any member of the family Planariidae and related families.

      Most planarians occur in fresh water and are sometimes seen in large masses; some species are marine, others are terrestrial. Some species are parasitic; i.e., they obtain nourishment from the body of another living animal.

      The body, when elongated, is soft, leaf-shaped, and ciliated. The spade-shaped head has two eyes and sometimes tentacles. The tail is pointed. The mouth is on the ventral, or lower, side, often more than half-way toward the tail. A body cavity, or coelom, is absent. The pharynx, which may be protruded from the mouth, ends in an intestine that is usually blind. The length is usually about 3 to 15 mm (0.1 to 0.6 inch); some grow to more than 30 cm (about 1 foot) long. Tropical species are often brightly coloured. Members of the North American genus Dugesia are black, gray, or brown.

      Planarians swim with an undulating motion or creep like slugs. Most are carnivorous night feeders. They eat protozoans, tiny snails, and worms. All are simultaneous hermaphrodites; i.e., functional reproductive organs of both sexes occur in the same individual. The reproductive organs begin to develop in early autumn. Cocoons containing fertilized eggs are laid in spring. In most species, fully developed young emerge and develop without metamorphosis (i.e., radical change), but free-living, ciliated larvae are released in a few marine species. In some species, the organism in the cocoon divides into two parts, each of which develops into a complete individual. New individuals, called buds (bud), form at the tail end of others in the genus Microstomum and may remain attached to the parent for some time; chains formed of three or four buds sometimes occur. Because of their remarkable ability to regenerate lost parts, planarians are often used experimentally to study the process of regeneration.

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Universalium. 2010.

Look at other dictionaries:

  • Planarian — Pla*na ri*an, n. (Zo[ o]l.) One of the Planarida, or Dendroc[oe]la; any turbellarian worm. {Pla*na ri*an}, a. [1913 Webster] …   The Collaborative International Dictionary of English

  • Planarian — Pla*na ri*an, n. (Zo[ o]l.) One of the Planarida, or Dendroc[oe]la; any turbellarian worm. {Pla*na ri*an}, a. [1913 Webster] …   The Collaborative International Dictionary of English

  • planarian — [plə ner′ē ən] n. [< ModL Planaria, name of the genus < LL planarius, flat < L planus,PLANE2] any of an order (Tricladida) of small, soft bodied, free living, turbellarian flatworms moving by means of cilia …   English World dictionary

  • Planarian — Polycelis felina, a freshwater planarian Planarian is the common name given to many non parasitic flatworms of Turbellaria class.[1] It is also the common name for a member of the genus Planaria within the family Planariidae. Sometimes it also …   Wikipedia

  • planarian — pla·nar·i·an ē ən n any of the family Planariidae or order Tricladida comprising small soft bodied ciliated mostly aquatic turbellarian worms planarian adj * * * pla·nar·i·an (plə narґe ən) any of the free living flatworms of the class… …   Medical dictionary

  • planarian — noun Etymology: New Latin Planaria Date: circa 1858 triclad; especially any of various dark colored freshwater triclad flatworms (family Planariidae) with two eyespots and a triangular head …   New Collegiate Dictionary

  • planarian — noun Any of various freshwater flatworms, of the family Planariidae. See Also: planar …   Wiktionary

  • planarian —    A flatworm. A relatively simple wormlike animal with a flattened ribbonlike body, a distinct head end, and a mouth located more or less centrally on the underside of the body [23] …   Lexicon of Cave and Karst Terminology

  • planarian — plÉ™ neÉ™rɪən n. type of flatworm which lives in water …   English contemporary dictionary

  • planarian — [plə nɛ:rɪən] noun Zoology a free living aquatic flatworm. [Order Tricladida.] Origin C19: from mod. L. Planaria (genus name), feminine of L. planarius lying flat …   English new terms dictionary


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