Planaria are small, free-living worm-like organisms commonly referred to as flatworms due to their nearly two-dimensional dorsoventrally-flattened appearance..

The following traits are important for understanding the role of planaria in the evolution of higher organisms:

Triploblast tissue organization (in development, body develops from 3 germ layers)

Bilaterally symmetrical body plan (body is divisible only along one central axis, into mirror image left and right halves)

Cephalization- sensory modalities concentrated in the anterior region, with a centralized nervous system including a primitive "brain" and two ventral nerve cords running parallel anterior to posteriorly..

Combined, these attributes allow the seemingly primitive planaria the ability to more effectively interpret environmental stimuli and move in a directed fashion. Extant planaria are attributed as being the living descendants of the first truly bilaterally symmetrical organisms with cephalization.