Commonly known as flat worms, they are acoelomate, triploblastic and bilaterally symmetrical animals which have no skeleton, respiratory and circulatory systems. Alimentary canal when present is in blind sac plan. They are unsegmented and soft-bodied invertebrate animals having flattened shapes that help in diffusion of oxygen and nutrients through their bodies.
Class TURBELLARIA(Dugesia; Bipalium; Leptoplana; Notoplana).
They are free living eddy worms, with flattened body and mouth located on the ventral side. Suckers are absent and epidermal cells are multiciliated. They are hermaphrodite. There are 4500 species, most of which are marine. Most turbellarians are carnivores and prey on small invertebrates or protozoans or scavenge on dead animals. One pair of eyes is generally present. All turbellarians are hermaphrodite, having both female and male reproductive organs and fertilization is internal. They have a brain and central nervous system, a well-developed excretory system and a complex reproductive system.
Commonly known as flukes, they are parasitic animals with suckers. Alimentary canal is highly branched, with mouth as the only opening. They are all internal parasites having leaf-like shape. They have suckers for attachment and the adults feed on the blood of host. This class is characterized by a complex hermaphroditic reproductive system and a life cycle that involves intermediate and main hosts. Like most internal parasites they have a very high reproductive rate, which is further increased by the larval stages which are also able to reproduce asexually, thereby creating even more new individuals.
Subclass MONOGENEA(Polystoma; Polystomoidella; Gyrodactylus; Dactylogyrus).
Aquatic parasites of aquatic vertebrates, they are dorsoventrally flattened animals with a posterior organ of attachment called haptor. The anterior sucker is often termed as prohaptor. There is no intermediate host. Small parasitic flatworms mainly found on skin or gills of fishes, they are rarely larger than 2 cm. Monogeneans lack respiratory, skeletal and circulatory systems and have none or weakly developed oral suckers.
Subclass ASPIDOGASTREA(Cotylaspis; Aspidogaster; Cotylogaster).
Suckers cover the entire ventral surface, with a number of small alveoli. Tegument bears small microtubercles. Nervous system shows extraordinary complexity. Gut with a single caecum. Hermaphrodite with one testis. Endoparasitic in the gut of molluscs, fish and reptiles. Hosts one or two. There are 4 families in this subclass which include about 80 species
Subclass DIGENEA(Fasciola; Schistosoma; Fasciolopsis; Opisthorchis; Paragonimus).
They are endoparasites of vertebrates having more than ten thousand species. Development involves two hosts and two generations. Larva is called miracidium. Tegument is syncytial, covered with cuticle. They occur worldwide and range in size from about 5 mm to 10 cm in length.
Flukes parasitize members of all vertebrate classes. Most are flattened, leaf-like or ribbon-like. Suckers on the ventral surface and hooks and spines are used for attachment. As there is no coelom, body is filled with mesenchymal connective tissue that surrounds all the body organs.
They are commonly known as tapeworms. Body is long, flattened and tape-like, with large number of proglottids or sometimes without segmentation. No digestive, respiratory and circulatory systems. They have saprozoic method of nutrition uptake.
Subclass CESTODARIA(Gyrocotyle; Amphilina).
No scolex and no division of body into strobila. Larva is called Lycophore which has 10 hooks. Sucker or adhesive organ is present on the posterior side. No digestive system and a single set of reproductive organs.
Subclass EUCESTODA(Taenia; Phyllobothrium; Haplobothriium; Echinococcus).
Body is divided into scolex, neck and strobila containing many proglottids. Larva is called hexacanth because it bears 6 hooks. They are intestinal parasites having many sets of reproductive organs. Digestive system is absent.