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[pluh-nair-ee-uh n] /pləˈnɛər i ən/
noun, Zoology.
any of various free-swimming, mostly freshwater flatworms of the class Turbellaria, having an undulating or sluglike motion: popular in laboratory studies for the ability to regenerate lost parts.
Origin of planarian
1885-60; < New Latin Planari(a) a flatworm genus (noun use of feminine of Late Latin plānārius level, on level ground; taken to mean “flat”; see plane1, -ary + -an Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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Examples from the Web for planarian
Historical Examples
  • The planarian worms that I have just been telling about are monsters.

    Natural Wonders Edwin Tenney Brewster
  • Its like the extra legs and tails on the lizard, and the extra heads on the planarian.

    Natural Wonders Edwin Tenney Brewster
  • Instead, much like the planarian cut in halves, it forms a whole animal, half size.

    Natural Wonders Edwin Tenney Brewster
  • Flat against the stone and not easily perceived may be a chiton, a planarian worm, or a nudibranch.

    The Sea-beach at Ebb-tide Augusta Foote Arnold
  • It is now nearly forty years since the presence of chlorophyl in certain species of planarian worms was recognized by Schultze.

  • Among the lowest forms of worms are the planarian worms, already alluded to as examples of the third grade of animal existence.

  • These are planarian worms, and though not of prepossessing appearance generally, are extremely interesting animals to study.

British Dictionary definitions for planarian


any free-living turbellarian flatworm of the mostly aquatic suborder Tricladida, having a three-branched intestine
Word Origin
C19: from New Latin Plānāria type genus, from Late Latin plānārius level, flat; see plane1
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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planarian in Science
Any of various small, chiefly freshwater flatworms of the class Turbellaria, having soft, broad, ciliated bodies shaped like a leaf. Planarians have a mouth on their lower side that is often closer to the tail than the head, and a three-branched digestive cavity. If a planarian is cut into several pieces, each piece can grow into a whole new organism.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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