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