Platyhelminthes
Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals. Unlike other bilaterians, they have no body cavity, and no specialized circulatory and respiratory organs, which restricts them to flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The Platyhelminthes have no skeleton and no extra body cavities giving them a flat unsupported shape.
Cestoda
Cestoda is the name given to a class of parasitic flatworms, commonly called tapeworms, of the phylum Platyhelminthes. Its members live in the digestive tract of vertebratesas adults, and often in the bodies of various animals as juveniles. The Cestoda does not have a skeletal system therefore the Cestoda lies in the shape of its skin and muscles.
Trematoda
Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic flatworms, commonly referred to as "flukes". Trematodes are flattened oval or worm-like animals, usually no more than a few centimetres in length, although species as small as 1 millimeter and as large as 7 centimeters are known. Their most distinctive external feature is the presence of two suckers, one nearby to the mouth, and the other on the underside of the animal. Trematoda have no skeletal system and they lie flat like all other Platyhelminthes.
Monogenea
Monogenea are small parasitic flatworms that are mainly found on skin or gills of fish. They are rarely longer than about 2 cm. A few species infecting certain marine fish are larger and marine forms are generally larger than those found on fresh water hosts. Monogeneans lack respiratory, skeletal and circulatory systems and have no or weakly developed suckers. Monogenea attach to their hosts using hooks, clamps and a variety of other specialized structures. They are often capable of dramatically elongating and shortening as they move.