The habitats of flatworm species vary widely. Some flatworm species are free-living and live in fresh, shallow water. However, nearly half of flatworm species are parasitic, meaning they need to live in a host organism to survive.
Flatworms are unsegemented, worm-like invertebrates. The free-living species prefer the underwater areas around rocks and aquatic plants where they feed on small organisms such as nematodes and protists. Parasitic flatworms can affect many species, including humans, cows, pigs and cats. Flatworms may cause serious disease in their hosts, posing a threat to humans and livestock alike. The lifecycles of parasitic flatworms take place within the host organism's bloodstream or digestive tract.