Introduction to the Platyhelminthes
Brief Taxonomy of Platyhelminthes
Basic Body Plan
The Tegument
Reproductive System
Digestive System
General Lifecycle
Introduction to the Platyhelminthes
This page is designed to illustrate the main features of the Platyhelminthes as a group, by comparing the features found within the different classes within this phylum. Features which are common between all of the classes will be emphasised, as will points where the different groups diverge.
The Platyhelminthes are dorso-ventraly flattened worms, with solid acoelomate bodies, (i.e. no body cavities), the organs and muscle fibres being embedded in parenchymal tissue. They also exhibit bilateral symmetry, and have defined posterior and anterior regions. Characteristically there is no respiratory or circulatory systems present. The most studied platyhelminthes are parasites, but there are also many free living members of the class Turbellaria. With the important exception of the schistosomes, they are mostly hermaphroditic, their reproductive system showing many features which are common to a number of the different main groups. 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.