Helminth Taxonomy - Introduction to the Phylum Platyhelminthes (Flatworms)
The Platyhelmithes are dorso-ventraly flattened worms, with solid acoelomate bodies, (i.e. no body cavities), the organs and muscle fibres being embedded in parenchymal tissue. There is no respiratory or circulatory systems present. They are mostly parasites, except for free living members of the Class Turbellaria, and with the important exception of the schistosomes, are mostly hermaphroditic. The outer layer of the body consists of a biologically active syncytial layer called a tegument, which varies in structure between the different classes of Platyhelminthes, (for example the tegument of many of the free living Turbellarians is ciliated, whilst that of the Cestodes is covered in numerous microtriches, and that of the Digeneans is covered in larger spines). The Platyhelminthes are divided into four classes. The Class Turbellaria is diverse class, containing free living, commensal and parasitic groups, but in terms of human or veterinary pathology is not very important, (most of the parasitic forms being found in invertebrates). The Class Monogenea is also only of limited importance here, although monogeneans may be important parasites of fish, and may cause severe disease in fish farms where there is a high density of potential hosts. The two most important classes in terms of human or veterinary pathology are the Class Trematoda, which may be further divided into the Sub Class Digenea (which include the schistosomes), and Sub Class Aspidogastrea, and the Class Cestoda, divided into many sub classes, the most important being the Subclass Eucestoda.