Flatworm is a type of worm known for the flat body shape of many species (kinds)—especially larger ones. Some flatworms live freely on land or in water. Others live as parasites in human beings or other animals.
Flatworms have a simple body structure. A layer of cells called the epidermis covers the animal’s body. In most flatworms, a mouth on the head, rear, or underside opens into a simple, sacklike intestine. A tightly packed mass of cells called parenchyma fills the body between the epidermis and intestine and contains the reproductive organs. Muscles, glands, and major nerves lie under the epidermis near the parenchyma.
Many flatworms have a smooth, soft body. Some flatworms have suckers or other projections on the body. Parasitic species use them to attach to their host, the organism on which they live. Some flatworms have spines and tiny, needlelike spicules that serve as a kind of skeleton. Most flatworms measure less than 1 inch (2.5 centimeters) long. But the largest flatworms, called tapeworms, may grow up to 100 feet (30 meters) long.
There are tens of thousands of species of flatworms. The largest group of species, called Neodermata, includes monogeneans, trematodes, and tapeworms. These parasitic flatworms live in a wide variety of hosts, usually vertebrates (animals with backbones). The rest of the flatworms are turbellarians, mostly free-living species found in sand or mud or on underwater plants. A few species live on land in moist soil.
Almost all flatworms are hermaphroditic—that is, the same individual has both male and female reproductive organs. Most turbellarians lay eggs that hatch into young resembling tiny adults. In other turbellarians, and in all parasitic flatworms, the young—called larvae—look different from adults and live in different habitats. For example, the larva of a monogenean has hairlike cilia that enable it to swim. The larva swims until it finds an appropriate fish for a host. The larva attaches to the fish and develops into an adult. The adult, which lives on the skin and gills of the fish, lacks cilia and cannot swim.
Parasitic flatworms cause disease in their hosts. Schistosomiasis, for example, is a tropical disease caused by trematodes called schistosomes or blood flukes living in the blood vessels of the abdomen. Adult tapeworms that live in the intestine of human beings do not usually cause much harm. But tapeworm larvae cause serious diseases that can be fatal if not treated.
