Platyhelminthes
The members of Platyhelminthes are Nematoda, and Rotifera have a vermiform or wormlike shape. They are also the first to have third layer of cells, the mesoderm, instead of the epidermus and gastrodermis. The mesoderm in they're bodies produces muscular layers, an excretory system and a reproductive system. They have complex body cavitys that hold specialized organs, and the fluid for the cavitys act like an hydrostatic skeleton. Flatworms have their bodies loosly filled with cells and have a acoelomate body plan. The cells that fill them up, are called parenchyme, witch is one way of organizing they're body structure. Another way is a plan called pseudocoelomate body plan, witch has a fluid filled cavity full of pseudocoelom. The last body plan is the coelomate body plan, they have they're tissues lined with mesodermal tissue.
Parasites
They also have many parasites, but not all are extremely dangerous. The most watched out are, Trematoda, Cestoda, often called tapeworms as well, and Digenea, also called Flukes.
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