planarian

American  

[pluh-nair-ee-uhn] / pləˈnɛər i ən /

noun

Zoology.

1. 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.

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planarian British  

/ pləˈnɛərɪən /

noun

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

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planarian Scientific  

/ plə-nâr′ē-ən /

1. 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.

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Etymology

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”; plane ¹, -ary + -an

Example Sentences

Adult planarian stem cells can transform into any type of cell, unlike most animals' stem cells, which are carefully restricted to forming only a few cell types.

From Science Daily • Oct. 29, 2025

Even a small part of a planarian can regenerate into a full worm with the typical shape and proportions.

From Scientific American • May 31, 2023

María Lucila Scimone, a researcher at M.I.T.’s Whitehead Institute for Biomedical Research, first noticed these cells while studying Schmidtea mediterranea, a planarian common to bodies of freshwater in Southern Europe and North Africa.

From New York Times • Jun. 29, 2020

LoCascio, S. A., Lapan, S. W. & Reddien, P. W. Eye absence does not regulate planarian stem cells during eye regeneration.

From Nature • Nov. 21, 2017

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

From Country Walks of a Naturalist with His Children by Houghton, W. (William)