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common name for several genera of the free-living (turbellarian) flatworms belonging to the order Tricladida, a name that derives from their characteristic three-branched digestive cavities. Most species range from 1-8 in. to about 1 in. in length (.32–2.54 cm) although some giant tropical forms range up to 2 ft. (60 cm). The different species are generally white, gray, brown, or black; a few forms are transparent. Many are striped or streaked and some of the large terrestrial species are brightly colored. Although planarians can be found in marine or moist terrestrial habitats, most inhabit freshwater areas. They crawl about over a trail of mucus that is secreted by specialized epidermal cells; the smaller forms move about by means of cilia on their ventral, or lower, surface, and larger species utilize muscular contractions as well. Tactile and chemoreceptive cells, located in the epidermis, serve as general sense organs. In many species these cells are clumped in lobes at the sides of the head. Most planarians are also light-sensitive and in some, pigmented light-sensitive cells are clumped in two cups that serve as primitive eyes. Planarians are usually either carnivorous or scavengers. The mouth is located near the middle of the ventral surface. The tubelike pharynx can be everted from the mouth and inserted into the prey; food is partially digested externally before it is sucked into the gut. Planarians are hermaphroditic; each individual worm contains both male and female organs, and, most commonly, they reproduce sexually. However, species similar to the 1/2-in.-long (1.27-cm) Dugesia tigrina, the most common planarian in the United States, are much studied in classrooms and laboratories for their additional capacity to reproduce asexually by transverse rupture of the body: a rupture line develops behind the mouth, and while the back half of the worm is anchored, the front half moves forward until the worm snaps in half. Each half regenerates the missing parts. Such planarians can also regenerate parts that are cut from the body. Planarians are classified in the phylum PlatyhelminthesPlatyhelminthes
, phylum containing about 20,000 species of soft-bodied, bilaterally symmetrical, invertebrate animals, commonly called flatworms. There are four classes: the free-living, primarily aquatic class, Turbellaria, and Trematoda, Cestoda, and Monogenea, which are
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, class Turbellaria, order Tricladida.
References in periodicals archive ?
This is a remarkable and very rare planarian, of unusually large size, and with black and white markings.
Human counterparts exist for most of the genes that we have found to regulate the activities of planarian stem cells," said Sanchez Alvarado.
Reproductive ecology and evolution in the invasive terrestrial planarian Bipalium adventitium across North America.
This fact is an important neuroevolutionary advance, since we did not observe inhibitory synapses in planarians, neither in annelids (Palacios-Pru et al.
Bipalium adventitium Hyman, 1943 (Turbellaria: Tricladida: Terricola) is an exotic terrestrial planarian that feeds on earthworms (Dindal, 1970; Ogren, 1981) and is invading North America (Hyman, 1954; Ogren, 1984; Ducey and Noce, 1998).
Streng for numerous discussions about planarian morphology and reproduction, L.
A pluralistic version of utilitarianism may also justify the intuition that, other things being equal, it is better to use a planarian than a mouse, and better to use a rat than a chimpanzee.
In a previous study we demonstrated that a complex process of early autophagy, followed by apoptotic processes, occurs in the hyperplasic ovary of the freshwater planarian Dugesia arabica.
Wright State University Is Currently Soliciting Quotes From Vendors Who Supply Confocal Microscopes That Meet The Following Objective: The Objective Of This Purchase Is To Be Able To Obtain Fluorescent Images Of Confocal Sections Through 1 Mm Of A Planarian Flatworm.
We analyzed planarian behavior when they were exposed to two test solutions that might be more realistic, a 10-fold dilution of the full-strength solution and a "poked" test solution in which an animal was placed in 1 ml of water and then stabbed 10 times with a pin.
The results obtained indicate that the planarian Dugesia gonocephala from the Chilean Altiplano is an excellent biosensensor (biomarker) to determine toxic effects on reproductive ecotoxicology.