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A common flatworm that lives in moist soil and fresh water. Many species are hermaphroditic, reproducing sexually. Most species range in size from 1/8 of an inch to an inch, but there are tropical forms which can grow as large as two feet long. Planaria have eye spots that are sensitive to the presence of light, but do not see images. One species that is commonly studied is Dugesia tigrina.

Dugesia tigrina and similar species can reproduce sexually or asexually. To reproduce asexually, it anchors the back half of its body while the front half moves forward. This leads to it's body being broken in half. Each end regenerates it's missing end.

On a side note, in high school biology I did an experiment on the effects of caffeine on the rate at which planaria reproduced asexually. The results were inconclusive and terribly unscientific, but it did seem that the caffeinated group split slightly more often than the control group. For other studies on planarian worms of dubious scientific integrity, see: the ability of planarian worms to run a maze more successfully after being fed the remains of a successful worm.

Pla*na"ri*an (?), n. Zool.

One of the Planarida, or Dendrocela; any turbellarian worm.

-- Pla*na"ri*an, a.


© Webster 1913.

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