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planarian

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 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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Turbellaria

Turbellaria A class of free-living flatworms (see Platyhelminthes) comprising the planarians, which occur in wet soils, fresh water, and marine environments. Their undersurface is covered with cilia, used for gliding over stones and weeds. Planarians can also swim by means of undulations of the body.

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planarian

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Turbellaria

Turbellaria

The class Turbellaria is the most primitive group within the phylum Platyhelminthes, the flatworms. Turbellarians share some important characteristics with other Platyhelminthes. All flatworms are flattened dorsoventrally . They are bilaterally symmetrical , are unsegmented, and are acoelomates , which means they do not have a body cavity. Turbellarians are solid because all the space around their digestive cavity is filled with muscle and other tissue. Turbellarians do not have a respiratory or circulatory system, they exchange gases by diffusion through all their cells. They have a muscular mouth, called a pharynx, as well as a saclike digestive cavity. Turbellarians also have an osmoregulatory system called the protonephridium. This system is made up of tubules, a network of little tubes, and specialized cells called flame cells.

The turbellarian nervous system includes a primitive brainlike structure in the head region, called a ganglion. This ganglion is formed by the thickening of the anterior part of the ventral nerve cords. The head region also has specialized sensory organs, which are more complex in turbellarians than in other flatworms. These organs include eye spots, which are composed of photoreceptors that detect light and are tactile and chemical sensory organs that help turbellarians find food. Movement is assisted by receptors that help maintain balance as well as detect movement. The sensory organs of turbellarians are more complex than those of other flatworms because turbellarians are free-living; all other flatworms are parasitic. The free-living turbellarians are ancestors of the parasitic flatworms; parasitism evolved as a specialized form of feeding and reproducing from the scavenger lifestyle of turbellarians. Turbellarians eat both living and dead animal material. Some turbellarians secrete digestive enzymes onto their food, then ingest the already-digested food particles through their pharynx. Others digest food in their digestive cavity. All flatworms must expel undigested food out of their mouth; they do not have an anus.

Turbellarians move around using cilia on their epidermis or by undulating their body with their muscles. Most turbellarians live in water, either fresh or salt water. A few species live on land in damp habitats like leaf litter. Turbellarians reproduce by fission and regeneration, or sexually. Turbellarians that reproduce sexually are hermaphroditicsperm from one animal will fertilize eggs from another, and the eggs then hatch into small turbellarians. When reproducing by fission and regeneration, the tail end of the individual turbellarian adheres to a substrate and the head region pulls away from the tail. This eventually splits the flatworm in two, and each piece regenerates the end that is missing. Turbellarians are the only flatworms that can reproduce by fission.

There are more than 4,500 species of turbellarians. Most are less than 5 millimeters (0.2 inches) long, and many are microscopic in size. Planarians (Dugesia ) are largest turbellarians; they can grow up to 0.5 meter (20 inches) long.

see also Phylogenetic Relationships of Major Groups.

Laura A. Higgins

Bibliography

Anderson, D. T., ed. Invertebrate Zoology. Oxford: Oxford University Press, 1998.

Barnes, Robert D. Invertebrate Zoology, 5th ed. New York: Saunders College Publishing, 1987.

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Purves, William K., Gordon H. Orians, H. Craig Heller, and David Sadava. Life: The Science of Biology, 5th ed. Sunderland, MA: Sinauer Associates Inc. Publishers, 1998.

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Turbellaria

Turbellaria (phylum Platyhelminthes) A class of worms, most of which are not parasites. The epidermis of most is ciliated. Almost all have a cavity for the gut and a ventral mouth with a well-developed pharynx.

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