Flatworm

Flatworm, common name for soft-bodied, usually parasitic animals, the simplest of animals possessing heads. They are bilaterally symmetrical and somewhat flattened, and most are elongated. Three main classes are included in the flatworm phylum: tapeworms, which in the adult stage are parasitic in the digestive tracts of animals; flukes, which are parasitic in various parts of different animals; and planarians, which are free-living and non-parasitic. Some authorities also include a group of unsegmented marine worms; other authorities consider them a separate phylum.

The ectoderm (outer surface) of the free-living flatworms is usually covered with hair-like cilia; in the parasitic forms the ectoderm usually secretes a hardened material called cuticle. A well-developed musculature, found directly under the epidermis (skin layer), allows the body to expand and contract, thus changing the body shape to a remarkable degree. Vivid pigmentation is sometimes present in the free-living forms, but the parasitic forms are usually unpigmented. Flatworms have no true body cavity; the spaces between the organs are filled with a compact connective tissue called parenchyma. Except in the simplest forms, one end of the body (the head) is more specialized for sensory perception, and locomotion takes place in the direction of specialization. The oral and genital openings are on the ventral (under) side. When present, the digestive tract is either sac-like or branched and has only one opening. This opening may be equipped with a sucker, as in the flukes, or, as in most planarians, it may have a well-developed pharynx. The nervous system consists of a network with a large ganglion (brain), at the front of the body, and various longitudinal nerve cords forming the principal parts. Sensory cilia and “eye spots” may be present in the free-living forms and in the larvae of the parasitic forms. The flatworm has no blood or vascular system. Specialized cells possessing cilia, called flame cells, lead from the interior to one or more openings in the exterior by means of a network of tubes. Together these structures form the excretory system. The reproductive system is highly complex and occupies a large portion of the interior of the animal. Although flatworms are almost all hermaphroditic (both male and female reproductive organs are present in each individual), the eggs and sperm are formed separately. These germ cells either leave the body by separate openings or enter a common chamber, called the genital atrium. Many flatworms are also able to reproduce asexually both by binary fission—that is, by pinching themselves apart to become two—and by regeneration, producing an entire new worm from a piece that has been cut off.

Free-living flatworms are found in almost every kind of environment, on land and in fresh and salt water. They eat a variety of foods, including plankton, carrion, earthworms, snails, insect larvae, and small crustaceans. The parasitic flatworms often display a complicated life cycle, which typically requires development in two or three hosts before completion.

Scientific classification: Flatworms constitute the phylum Platyhelminthes; tapeworms constitute the class Cestoda, flukes the class Trematoda, and planarians constitute the class Turbellaria.