Platyhelminthes, or Flatworms, are the simplest kind of worms. There are over 20,000 species of platyhelminthes. They are found all types places including in the water, dirt or living off of different types of animals. They can be free living or parasitic latching on to a host. A parasite lives off of another living thing called a host and can be harmful to the host or can actually benefit the host in rare occasions. One of the most known flatworms is the tapeworm. Flatworms are found in marine and fresh water.