The flatworms are also known in scientific literature as Platyhelminthes. They are the simplest form of the worm groups. There are about 20,000 species in this group. They are found in many places mainly in marine and fresh water. The can be harmful too. They are considered as prototypes on having developed such innovations as bilateral symmetry, a head, tail, and three germinal tissue layers which are stinging celled animals, comb-jellies, only have two. These simple soft-bodied animals use their skin to breath by diffusion through and only have one body opening, the mouth serving also as an anus. These are also important species to humans directly and indirectly through their negative interactions with food and ornamental animal life.