The Phylum Platyhelminthes consists of bilaterally symmetrical free-living parasitic unsegmented worms (the Cestoda excepted that are strobilated, or segmented) usually dorso-ventrally flattened. There is marked cephalisation with a central nervous system; usually there is a complex but incomplete gut present including a mouth, but an anus is absent; protonephridia serve as excretory organs. The body is acoelomate and triploblastic; the reproductive system is typically hermaphroditic.
Five classes are recognised. The Class Turbellaria contains the free-living flatworms, the other four classes are all entirely parasitic and their hosts include many marine species, as fish, mollusks and larger crustaceans.
[After Hayward and Ryland, 1995; Brusca and Brusca, 1990]