Any of about 3,000 species (class Cestoda, phylum Platyhelminthes) of bilaterally symmetrical parasitic flatworms found worldwide. Tapeworms range from 0.04 in. (1 mm) to more than 50 ft (15 m) long. The head bears suckers and often hooks for attaching to the liver or digestive tract of the host. Once attached, a tapeworm absorbs food through its body wall. The body is often divided into a head, or scolex, possessing the suckers and hooks, an unsegmented neck, and a series of proglottids (units containing both male and female reproductive organs) that continually form in a growth region at the base of the neck. Following fertilization, each mature proglottid containing thousands of embryos breaks off and is eliminated in the host's feces. If these are ingested by an animal (the intermediate host) grazing on food contaminated with feces, they develop into larvae, which bore through the intestinal wall into the circulatory system and are carried to muscle tissues, which they burrow into, forming a dormant cyst. When meat is inadequately cooked, humans become infested with the larvae, which attach to the intestinal wall. Many species that infest humans belong to the genus Taenia; the intermediate host is implied by the name (e.g., beef tapeworm, T. saginata). Humans can also acquire tapeworms through fecal contamination of soil or water.
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