EMIGRATION CREEK PROJECT Ecology 340 Westminster College, Salt Lake City, Utah Bridgette Jenne
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PLANARIAN (Dugesia)
Planarian. 1999-2000. Encyclopedia Britannica. http://www.britannica.com/seo/p/planarian/ Index: These small, free-living flatworms are found under rocks in the fresh water of Emigration Creek where it crosses the campus of Westminster College. They are classified in the phylumPlatyhelminthes and in the class Turbellaria. We have identified the Emigration Creek species as Dugesia tigrina, a common NorthAmerican flat worm. Planarians are generally zoophagous, feeding on small living invertebrates and decaying organisms found in their ecosystem. Emigration Creek planarians could eat segmented worms, leeches, snails, and other small herbivores. These small herbivores, in turn, eat the filamentous algae, Cladophora, and the diatoms growing in the "biofilm" of rocks. Planarians may play an role as a "detritivore" organism. Detritivore organisms gain their food energy with the help of decomposing fungi and bacteria which, with the help of various invertebrates, start the fragmentation process of materials that fall into the stream. A major source of energy input for this detritivore food chain is the leaves of the local riparian trees. The leaves are partially decomposed by bacteria and fungi which can then be eaten by Dugesia and other aquatic organisms.
The planarian eats by using its long, highly muscular pharynx. This protrudes through the mouth in the presence of food. The tip is placed against the food and soft or disintegrating particles of tissue are sucked up into the main gastrovascular cavity by the muscles in the pharynx. The cavity will be come full of fluid and small bits of tissue after about 30-80 mins of feeding. Individual particles are ingested by pseudopodial action of large gastrodermal cells. Planarians are usually between 3 to 15 mm. The colors seen in the North American genus, Dugesia, are white, grey, brown, and black. The triangular head contains two eye spots and sometimes tentacles for gripping. The mouth, or feeding structure called a pharynx, is located on the underside of the worm halfway down towards the tail.
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These soft bodied, ciliated flat worms move much like slugs in an undulating motion. The movie to the right shows a Dugesia tigrina from Emigration Creek moving under a dissecting microscope. |
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Dugesia tigrina reproduces both sexually and asexually. Under the appropriate conditions of temperature and food, asexual individuals pinch in around mid-length of the worm's body. The pinching continues until the two halves are complete. Each half will then form a whole worm. This aexual reproduction occurs only above 10degrees C with the frequency of division increasing to a maximum at 25 to 28 degrees C. Sex organs may develop during the winter and early spring in some strains of D. tigrina. Planarians are hermaphroditic in that organs for both sexes develop on each worm. Egg capsules, or cocoons that are oval in shape and about 5 mm long, are deposited in May and June. As the water temperature rises the reproductive organs degenerate and in July, August, and September reproduction may be entirely by fission. By late autum the sex organs begin to develop again. Fully developed young emerge and develop without metamorphosis. Planarians are also capable of reproducing body parts if they have been damagedor removed. This has made them very popular in laboratory experiments. |
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REFERENCES: 1. McCafferty, Patrick W. 1981. Aquatic Entomology. (p.74). Arthur C. Bartlett, London 2. Pennak, Robert W. 1978. Fresh-water Invertebrates of the United States: Second Edition. (p. 116-131). John Wiley & Sons, Inc., USA. 3. Planarian. 1999-2000. Encyclopedia Britannica. (http://www.britannica.com/seo/p/planarian). 4. Planarian. 2000. The Columbia Electronic Encyclopedia. |
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