Cestoid Worms

Cestoid Worms (Cestoda), an order of flat worms (Plathelminthes), of internal parasitic habit, and generally known as Tapeworms (q.v.). The adult consists of an asexual 'head,' attached by hooks or suckers or both to the host, and budding off a long chain of flat sexual, hermaphrodite 'joints,' which become mature at a certain distance from the 'head,' have a measure of individuality and independence, and are eventually expelled. There is no alimentary canal nor vascular system; the nervous system is usually complex, but of a low order; there is a well-developed excretory system of branching tubes. The reproductive organs of the 'joints' are usually very complex. The liberated 'joints' or 'proglottides' break up, and set free embryos, which find their way into other hosts, and undergoing considerable change become bladder-worms, develop a head, or in some cases heads, and only become sexual when their host is in turn eaten by the original species in which the tapeworm flourished. There is thus an alternation of generations between the asexual bladder-worm and the sexual tapeworm. The order includes about 25 genera and 500 species, mostly parasitic in vertebrates. The genus Tænia (tapeworm) includes more than half the known species. The Cestodes are linked to the flukes or Trematodes by forms like Amphilina, Caryophyllæus, and Archigetes, which have no 'joints,' and a single reproductive system; and there is a well-marked series from these up to the most specialised Tænia. Echinebothrium, Phyllobothrium, Anthobothrium, Acanthobothrium, Tetrarhynchus, Ligula (q.v.), Bothriocephalus (q.v.), are the important genera besides Tænia. See TAPEWORMS; also BLADDER-WORM, PARASITISM, and Leuckart's Parasites of Man.