The typical representatives of the phylum Platyhelminthes, planarians, consist of small flatworms, are usually about 0.5 inches in length, and occur with gray or black coloring (1:382). The planarians are highly important to the student of biology, given their illustration of certain advances in the evolutionary process over the coelenterates. The planarian is abundant over the entire world in those spring-fed streams or cool, clear, permanent lakes and streams that are uncontaminated, in which they feed on small living or dead organisms. The planarians avoid light by clinging to the undersurfaces of stones or logs in the water.
The planarian differs from the hydra or a medusa in several ways (1:385). First, it has bilateral symmetry rather than