PLATYHELMITHES- Platyhelminthes is a phylum of relatively simple unsegmented, soft-bodied invertebrate animals. Flatworms do not have a body cavity called a coelum. They have an incomplete digestive system and no specialized circulatory or respiratory organs. They must use their flat shape to allow oxygen and nutirants to pass through their skin to all parts of their bodies. Flatworm are the first of the simple animals to show bilateral symmetry. Bilateral Symmetry will divide an organism into mirror image halves. There is an approximate reflection symmetry. Often the two halves can be referred to as the right and left halves.
Tubelleria- There is about 4,500 species and are mostly free living. They are mostly predators or scavengers. But, some of these are symbiotes to other animals. The free-living tubellerians are mostly brown, black, and gray. The larger ones are light colored though. They also have no cuticle. most of these have small eyes. There are few of the larger species though that have many eyes. All of the tubelllerians are hermaphrodites. This means they have both male and females reproductive systems. The larger species mate by penis fencing which is a battle in which each tries to impregnate each other.
Digenea- These are sometimes called flukes. Most of them have flat rhomboid shapes like a flounder. They have about 11,000 species. This is more than all the other platyhelmithes. The adults usually have two holdfasts. these holdfasts consist of a ring around the mouth and a large sucker midway. The name digeneans means two generations most of them have very complex life styles. In the intermediate stage of the species the parasites transfer from one host cell to the other host cell.
Cestoda- This species is also called tape worms because of their flat, slender and long bodies. In latin the word cestus means tape. The majority of the species is known at eucestodes. The neck produces a chain of segments that are also called proglottids. They do this by the process of strobilation. Every one of the proglottids have both a male and a female reproductive organs. When the eggs are fully developed the proglottids separate and are then excreted by the host cell.