TAPEWORMS (Cestoidea or Cestodes), a class of endo parasitic Platyhelminthes (q.v.) in which the body is usually flattened and ribbon-like, and may be unisegmental or composed of a chain (strobila) of "segments" (proglottides). In the latter case the segmentation is not comparable with that of an annelid or arthropod, but is probably to be looked upon as an adaptive device for the multiplication and dissemination of eggs. The number of segments in a complete strobila varies, in different species, from three or four to many hundreds, and the length of the chain from less than a millimetre to several metres. Each segment contains a separate set of reproductive organs, but the nervous and excre tory systems are continuous throughout the chain. Tapeworms have no mouth or alimentary canal, liquid food being presumably taken in by absorption through the body-wall. The adult worms are, with few exceptions, intestinal parasites of vertebrates. In almost all known cases the life-history involves an intermediate host and a somewhat complex metamorphosis.
In the segmented forms there is, at what is usually called the anterior end, a specialized "head" or scolex, serving as an organ of fixation. This is generally provided with muscular suckers or variously modified sucker-like organs known as bothria or both ridia, and may, in addition, bear a central proboscis-like structure, the rostellum, which is commonly armed with chitinoid hooks. The region nearest to the scolex is the zone of proliferation of new segments, which are, as a rule, continually being formed throughout the life of the worm. As they pass back along the
strobila the segments become successively sexually mature and finally gravid, the most posterior usually containing little but a uterus crowded with eggs.
The nervous system consists of an intercrossing system of nerve-fibres and ganglionic cells at the anterior end (in the scolex in strobilate forms), and usually a single pair of main lateral nerve cords which are continuous throughout the body and without segmental ganglia. The excretory or "water-vascular" system is composed typically of two pairs (dorsal and ventral) of main longitudinal canals, continuous throughout the strobila and con nected together anteriorly.
The animals are, with rare exceptions, hermaphrodite, each in dividual, or each segment in strobilate forms, being potentially male and female. The male duct and the vagina may have separate apertures or may open side by side into a common atrium. There is considerable variation in the position of these openings. In unsegmented forms they may be terminal, subterminal or ven tral, while in segmented forms they may be on the ventral surface or on one of the lateral margins of the segment. The essential organs of the genital apparatus do not differ greatly from those of other groups of Platyhelminthes. The oviduct connected with the ovary and its associated glands, and with the inner end of the vagina, leads into a uterus in which the fertilized eggs are ac cumulated.