Phylum of Metazoa.
The Platyhelminthes include various dorsoventrally flattened animals that were consequently commonly described as flatworms which are typically bilaterally symmetrical. Their primary body cavity is filled with the so-called parenchyma consisting of connective tissue fibers and unattached or fixed cells of various types which are surrounded by body fluids. The platyhelminthes are thus acoelomate worms which usually lack a definite anus (or even lack an intestine as in tapeworms = cestodes), as well as skeletal, circulatory, and respiratory systems. They are covered by a typical, group-specific body wall (Tegument) through which an active uptake of nutrients regularly occurs; a cuticle of any kind (suggested in light-microscopic studies) is always absent and would hinder the feeding. Terminal cellsof the protonephritic type (cyrtocytes) function as an excretory system. Except for a few species, the parasitic...