planarian, 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 Platyhelminthes, class Turbellaria, order Tricladida.

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