Platyhelminthes are flatworms, a large number of which are found in the sea, either free-living or parasitic.
Class Turbellaria. Nearly all of this class are free-living on the bottom under stones and in crevices, where they move about by means of cilia covering the body.
Class Nemertinea. These are ribbonlike worms sometimes considered as a separate phylum. The benthic species live among rocks, algae, mussels, and so on, or burrow in the bottom, where they capture small organisms by means of a long eversible proboscis. Extraordinary size variations occur, some species being only 5 mm long, while one, Lineus longissimus, may become 25 m in length when extended, and therefore is the longest of the invertebrates; however, its threadlike form contains but little bulk. Fifty-two planktonic species of nemerteans are known, some living at great depths—for example, Pelagonemertes. (Coe, 1926). The planktonic forms are modified, some with caudal and horizontal fins for swimming (fig. 228c). There are about 550 species of nemerteans, of which nearly all are marine.