The Class Turbellaria of the Phylum Platyhelminthes consists of the free-living flatworms, predominantly aquatic and mostly marine. The body is unsegmented, the epidermis is cellular and usually ciliated. Most turbellarians creep more often than they swim. A few pelagic species are known from the marine environment, of which Alaurina composita (Order Macrostomida) is regularly found in the North Sea. Two other turbellarid species that possibly occur in the North Sea plankton are Plagiostomum vittatum (Order Prolecithophora) and Planocera pellucida (Order Polycladida).
[After Hayward and Ryland, 1995; Brusca and Brusca, 1990; Hansson, 1998d]
The following species of Turbellaria are included:
Order Macrostomida
Family Microstomidae
Alaurina composita
Order Prolecithophora
Family Plagiostomidae
Plagiostomum vittatum
Order Polycladida
Family Planoceridae
Planocera pellucida