[pluh-nair-ee-uh n]

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 Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for planarian

Historical Examples of planarian

  • 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.

British Dictionary definitions for planarian



any free-living turbellarian flatworm of the mostly aquatic suborder Tricladida, having a three-branched intestine

Word Origin for planarian

C19: from New Latin Plānāria type genus, from Late Latin plānārius level, flat; see plane 1
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

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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.