All the larger forms are flat with ribbon-like or leaf-like shapes, since their lack of respiratory and circulatory systems means that they have to rely on diffusion for internal transport of metabolites.

Traditional classifications divide the Platyhelminthes into four groups: Turbellaria and the wholly parasitic Trematoda, Monogenea and Cestoda.

The name "Turbellaria" refers to the "whirlpools" of microscopic particles created close to the skins of aquatic species by the movement of their cilia.

[5] As bilaterians, platyhelminthes are triploblastic,[4] but have no internal body cavity (are acoelomate), and lack specialized circulatory and respiratory organs,[4][5] so gas exchange is by simple diffusion.

Most are predators or scavengers, and terrestrial species are mostly nocturnal and live in shaded humid locations such as leaf litter or rotting wood.

They have many glands, usually submerged in the muscle layers below the skin and connect to the surface by pores through which they secrete mucus, adhesives and other substances.

[5] Small aquatic species use the cilia for locomotion, while larger ones use muscular movements of the whole body or of a specialized sole to creep or swim.

A few feed on larger animals, including oysters and barnacles, while some, such as Bdelloura, are commensal on the gills of horseshoe crabs.

[4] The freshwater species Microstomum caudatum can open its mouth almost as wide as its body is long, to swallow prey as large as itself.

[5] The intestine is lined by phagocytic cells which capture food particles that have already been partially digested by enzymes in the gut.

A few large species have many eyes in clusters over the brain, mounted on tentacles, or spaced uniformly round the edge of the body.

[5] All turbellarians are simultaneous hermaphrodites, having both female and male reproductive cells, and fertilize eggs internally by copulation.

[5] Some of the larger aquatic species mate by penis fencing, a duel in which each tries to impregnate the other, and the loser adopts the female role of developing the eggs.