Platyhelminthes very commonly known as flatworms or tapeworms, these animals are soft-bodied invertebrate animals. There are around 20,000 species of these animals. A few of these live as parasites on humans and other animals. It is because of this parasitic nature that they do cause some amount of trouble for the host animal. A few species belonging to this phylum can be a major cause of certain diseases. Schistosomiasis, or bilharzia or bilharziasis, is a disease caused by these parasitic flatworms belonging to the family Schistosomatidae.