The term Platyhelminthes “flatworms” was first proposed by Gaugenbaur (1859) and applied to the animals now included under that heading. The name Platyhelminthes has been derived from the Greek platys = flat + helmins = worms. At first, nemertines and others were also included but later were removed to other groups.
The phylum is now restricted to three classes, Turbellaria, Trematoda, and Cestoda. Although these classes have many structural differences, they all show enough similarity in the body pattern to indicate a common origin.
Platyhelminthes include the flatworms, their bodies are compressed dorsoventrally and show bilateral symmetry. They are the lowest triploblastic acoelomate Metazoa, but they are more advanced than Coelenterata because their tissues are organised into organs.
The mesoderm forms a type of connective tissue called parenchyma which fills the body spaces between the ectoderm and endoderm so that there is no coelom or haemocoele, hence, they are called acoelomate animals, mesoderm also forms organs, such as the excretory and reproductive organs.
The excretory system has one or two canals with branches, the finer branches end in flame cells, the canal has no internal opening but it opens to the exterior only. Blood vascular system and respiratory system are absent. There is no anus and in some even the mouth and alimentary canal are absent.
The nervous system consists of a network, but it has ganglia at the anterior end which serves as a brain. Reproductive organs are very highly developed, most Platyhelminthes are hermaphrodite.
The phylum includes some 15,000 species, and it is divided into three classes. Class Turbellaria includes ciliated flatworms which are generally free-living, Trematoda are non-ciliated parasitic flatworms or flukes, while Cestoda are all endoparasitic flatworms or tapeworms.
The typical structure of Platyhelminthes is seen only in Turbellaria, because the Trematoda and Cestoda, due to parasitic habit, have become different from their free-living ancestors, they have lost their ciliated epidermis and have acquired a cuticle and organs of attachment.
The trematodes have retained the body form and alimentary canal of Turbellaria, but the tapeworms have become elongated into a chain and the alimentary canal is lost.
