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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.
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References in periodicals archive ?
The findings may propel biological studies on highly regenerative organisms like planarians and also inform regenerative medicine efforts for other organisms like humans that have less regenerative capacity.
During the experimental period, each planarian was maintained at room temperature (21[degrees]C) under a 12 h dark-light cycle, in a clear plastic square dish (6 cm x 6 cm) filled with 9 mL APW.
Henriques, "Stress protein response and catalase activity in freshwater planarian Dugesia (Girardia) schubarti exposed to copper," Ecotoxicology and Environmental Safety, vol.
Notes On The Planarian Worms Obtained On The Upper Wellington
The ability to follow individual neoblasts opens the door to uncovering the molecular cues that help planarian stem cells navigate to the site of injury and ultimately may allow scientists to provide therapeutic stem cells with guideposts to their correct destination.
The team, from the University of Nottingham's School of Biology, is studying two types of planarian flatworms that can regenerate muscles, skin, guts and brains through a unique process of cell division.
Levin's group has taken the observation to a new level and has shown that manipulating electrical properties of cells can produce strange results, such as this four-headed planarian worm.
HAIL PLANARIAN! Eyed, flat, dark worm in the stream.