Platyhelminthes

Flatworm is a kind of worm. Some flatworms live freely on land or in water. Others live as parasites in human beings or other animals. Many flatworms, especially the larger species, have a flat body.

Flatworms have a simple body structure. A layer of cells called the epidermis covers the animal's body. An inner layer of cells forms an intestine in most flatworms. A tightly packed mass of cells called the parenchyma fills the body between the epidermis and intestine. Muscles, glands, nerves, and reproductive organs lie in the parenchyma. The only opening of the intestine is the animal's mouth. The mouth may be at the head end, the rear end, or the underside of the body.

Many flatworms have a smooth, soft body. Many have suckers or other projections on the body. 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. However, the largest flatworms, called tapeworms, may grow up to 100 feet (30 meters) long.

There are about 13,000 species of flatworms. They may be divided into four groups: (1) turbellarians; (2) monogeneans; (3) trematodes; and (4) cestodes, or tapeworms. Most turbellarians are free-living. They usually are found in sand and mud on the bottom of bodies of water. A few species live on land in moist soil. The other three groups of flatworms are parasites. They live in a wide variety of organisms that serve as hosts.

Almost all flatworms are hermaphroditic--that is, both male and female reproductive organs are found in the same animal. Most turbellarians lay eggs that hatch into tiny young that resemble the adults. In some turbellarians, and in all parasitic flatworms, young--called larvae--hatch from the eggs. The larvae look different from the 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 schistosomes (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. However, tapeworm larvae cause serious diseases that can be fatal if not treated.

Scientific classification. Flatworms make up the phylum Platyhelminthes. The four classes of flatworms are Turbellaria, Monogenea, Trematoda, and Cestoda.