The phylum Platyhelminthes embraces three classes of worms. Two of these are entirely parasitic. The Trematoda comprise flukes and the Cestoda, tapeworms. The third class, the Turbellaria, are free-living and are the ancestors of the two parasitic classes. The members of the phylum are dorso-ventrally flattened, accounting for the name flatworm, and display a solid, or acoelomate, grade of structure, in which there is no body cavity and mesenchyme fills the space between the internal organs and body wall. The mouth is the only opening to the digestive tract, where a digestive tract is present. On this CD-Rom, only the Turbellaria are dealt with.
The Turbellaria vary in shape from ovoid to elongate and, like the other flatworm classes, are usually dorso-ventrally flattened. In general, the larger the worm is, the more pronounced is the flattened condition. Head projections are present in some species. These may be in the form of short tentacles, which vary in number and position, or in the form of lateral projections of the head called auricles. Coloration is mostly in shades of black, brown and grey, although some groups display a brightly-coloured patterns, A few species are green, owing to the presence of symbiotic algae. Turbellarians range in size from microscopic forms to species that are more than 60 cm long, although most are less than 10 mm in length.
Turbellarians are primarily aquatic, and the great majority are marine. Although there are a few pelagic species, most of them are bottom dwellers that live in sand or mud, under stones and shells, or on seaweed. Many species are common constituents of the interstitial fauna. Freshwater forms live in lakes, ponds, streams, and springs, where they occupy bottom habitats. Some species have become terrestrial, but these are confined to very humid areas and hide beneath logs and leaf mold during the day, emerging only at night to feed.
Despite their similarity in appearance, turbellarians exhibit considerable internal complexity. The members of the order Polycladida exhibit a more primitive level of organization (often referred to as the archoophoran level) then the members of the order Seriata (referred to as the neoophoran level).
Source: Barnes, 1974.