Topics: Flatworm, Annelid, Platyhelminthes Pages: 2 (349 words) Published: May 6, 2007

A planarian is a small free-living flatworm. These flat worms are usually found under rocks in fresh water. They are part of the phylum Platyhelminthes and in the class Turbellaria.
Planarians are usually zoophagous, they feed on small living invertebrates and decaying organisms found in their community. Some planarians could eat segmented worms, leeches, snails, and other small herbivores. Planarians may also be organisms that gain their food energy with the help of decomposing fungi and bacteria. A major source of energy for the planarian's food chain is the leaves of the local riparian trees. These leaves are partially decomposed by bacteria and fungi.

The planarian eats by using its long, highly muscular pharynx. The tip of its pharynx is placed against the food and soft particles of tissue are sucked up into the main gustrovascular cavity by the muscles in the pharynx. After about 30-80 minutes of feeding the cavity will become full of fluid and small bits of tissue.

The common length of a planarian is between 3 to 15mm. The colors seen in the North America genus are commonly white, grey, brown, and black. The head is triangular, it contains two eye spots and sometimes tentacles for gripping. The pharynx is located in the underside of the worm halfway down towards the tail. Planarians move like slugs in an undulating motion.

Planarians reproduce both sexually and asexually. Under the right 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 form a whole worm. This type of reproduction only happens above 10 degrees Celsius to about 28 degrees Celsius. Planarians are hermaphroditic in that organs for both sexes develop on each worm. Eggs from planarians are deposited in May and June. In July- September reproduction may be entirely by fission. By late autumn the sex organs begin to develop again. Fully...
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