Tapeworms

cestodes, invertebrates of the class Cestoidea of the subphylum Platyhelminthes. In the adult stage, tapeworms are parasites of the human intestine and the intestines of other vertebrates. There are more than 3,000 known species. The body is ribbonlike, measuring from several millimeters to 20 m long (sometimes longer); it is usually divided into segments, or pro-glottids, which number from two to several thousand. At the anterior end is the head, or scolex, with true suckers, hooks, or bothridia and with one to four rostella and other organs of attachment. As the tapeworm grows, the neck forms the nonseg-mented part of the body, or the strobila, which is covered with a nonliving cuticle secreted by the subcuticula. Its surface bears microscopic nodules, or microtriches, which are important in food acquisition. There is no digestive tract; absorption of food is accomplished by the entire body surface.

Tapeworms are hermaphrodites. In the successively produced proglottids, male and female reproductive organs are formed; a uterus subsequently develops, which is filled with eggs. One or, less frequently, two or more reproductive systems develop in each proglottid. The production of eggs is usually prolific; for example, the beef tapeworm, which lives for 20 years, sheds up to 600 million eggs per year.

The life cycle of tapeworms is complex, involving a change of hosts (an exception is the dwarf tapeworm). The eggs are released to the exterior in the feces of the final, or definitive, host. A larva armed with three pairs of hooks (oncosphere or coracidium) usually develops in the uterus; this stage of development in some tapeworms is reached later, in the external environment. Subsequent development is associated with one or two intermediate hosts.

In cestodes that require only one intermediate host, the mature egg is swallowed by a polychaete, leech, crustacean, insect, mollusk, or vertebrate. In the stomach of the intermediate host the oncosphere leaves the egg and penetrates the body cavity or some organ, where it develops into a second larval stage, either a cysticercus, measle, coenurus, hydatid, or another type of larva. Coenuri and hydatids form many scolices as a result of asexual reproduction. The intermediate host is eaten by a vertebrate, which becomes the definitive host of the larvae. The larvae attach themselves with their scolices to the intestinal wall of the definitive host and grow into adult worms.

When development occurs in two intermediate hosts (as in the broad fish tapeworm), the first host (a crustacean) is infected by eating free-swimming coracidia, from which emerge oncospheres. The oncospheres penetrate the body cavity of a copepod and develop into the second larval stage, the procercoid. The procercoid, which is eaten by a fish (the second intermediate host), bores through the intestine into the body cavity or certain organs and tissues, where it develops into the next larval stage, the plerocercoid. The definitive host, man or some other vertebrate, receives the tapeworm by eating a fish infested with plerocercoids. In man, tapeworms cause hymenolepiasis, taeniasis, echinococcosis, and diphyllobothriasis.