Flatworm
Flatworms are one of the invertebrate groups which are studied in school biology. The main reason for this is that they may infect humans, pets and farmyard animals. Some do enormous harm to humans and their livestock. Schistosomiasis, caused by one genus of trematodes, is the second most devastating of all human diseases caused by parasites, surpassed only by malaria.  Effective treatments are now available for these infections. Also, in first-world countries, regulations on domestic cattle prevent infected meat from reaching the human hosts.