There are about 20,000 species of platyhelminthes, most of them parasitic, but only 20 species of free-living forms are known from the Gulf of California.
All flatworms are acoelomate, triploblastic and bilaterally symmetrical.
The turbellarians are considered the most primitive bilaterally symmetrical animals and are the first group to have evolved a true mesoderm from which all muscles have evolved. Indeed, all ceolomate animals may have evolved from flatworm-like ancestors. Most species have a conspicuous brain located in the anterior region of the body and connected to at least 1 pair of longitudinal nerve cords. All species are flattened dorso-ventrally, more or less. Each protonephridia consists of a group of cilia projecting into a delicate cup-like cell.