136 Tapeworm

Tapeworms are flatworms in the class Cestoda of the phylum Platyhelminthes. The body of an adult worm consists of a small head or scolex, which is usually armed with hooks or suckers to attach the animal to the wall of its host's small intestine, and a chain of segments or proglottids. New proglottids arise by budding from the scolex region. As they mature, they are pushed away from the head by the formation of new proglottids and develop both male and female sex organs. After fertilization, eggs or gravid proglottids are excreted with the host's feces. Tapeworm life cycles are complex. In general, the eggs must be ingested by an intermediate host, where they typically become saclike larvae in the tissues. When the host of the adult form (the definitive host) eats an infected intermediate host, adult worms develop in its intestine. Some species have two or more intermediate hosts and can use several species as the definitive host. Serious clinical disease often occurs when a parasite becomes established in an atypical host or when larval forms are able to develop in what is normally a definitive host.

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