Platyhelminthes, one of the first groups to evolve bilateral symmetry, are mainly small aquatic worms. Flatworms may be free living, such as the Turbellaria, or parasitic, such as the flukes and tapeworms. Possessing definite head and tail ends, but lacking true guts and circulatory systems, flatworms feed mainly on detritus or host tissue. Some of the worst known human parasites are flatworms.