Platyhelminthes

Abstract

A phylum of the invertebrates, commonly called the flatworms. They are bilaterally symmetrical, nonsegmented worms characterized by lack of coelom, anus, circulatory and respiratory systems, and exo- and endoskeletons. Many species are dorsoventrally flattened. They possess a protonephridial (osmoregulatory-excretory) system, a complicated hermaphroditic reproductive system, and a solid mesenchyme which fills the interior of the body. Some parasitic species, that is, some trematodes, have secondarily acquired a lymphatic system resembling a true circulatory system. Some species of trematodes, the schistosomes, have separate sexes. Traditionally, three classes were distinguished in the phylum: the Turbellaria, mainly free-living predacious worms; the Trematoda, or flukes, ecto- or endoparasites; and the Cestoda, or tapeworms, endoparasites found in the enteron (alimentary canal) of vertebrates, whose larvae are found in the tissues of invertebrates or vertebrates. However, recent cladistic analyses using morphology, including ultrastructure, as well as DNA analysis, have shown that the "Turbellaria" are an assemblage of taxa that are not monophyletic (that is, they are not a group containing all taxa with a common ancestor), and that the monogeneans, earlier included in the trematodes, do not belong to the trematodes. Most importantly, the Acoela do not belong to the Platyhelminthes, but are a very archaic group close to the base of the lower invertebrates; and all major groups of parasitic Platyhelminthes - that is, the Trematoda, Monogenea, and Cestoda - are monophyletic, constituting the Neodermata.