Transparency Master: General Tapeworm Structure

Tapeworms or cestodes (Class Cestoidea) are a particularly interesting and unique group of animals. All members of the class are endoparasites, and the tape-like body is an efficient adaptation for existence within the alimentary canal and related ducts of vertebrates. Cestodes lack a mouth and digestive tract. They are dorsoventrally flattened and possess minute projections or microtriches to facilitate absorption of nutrients. The body or strobila is composed of a head or scolex, possibly an unsegmented region referred to as the neck, and a series of segments. Although figure 1 is an obvious exaggeration, the length of some species may be surprising to the average person. The beef tapeworm, for example, commonly attains a length of 10 to 15 feet inhumans, and the broad fish tapeworm, also in humans, may exceed 50 feet. The scolex functions as an attachment or locomotor organ, and is adorned with suckers (fig. 3), bothria (grooves) (fig. 2), bothridia (leaf-like extensions) (figs. 5 and 7), hooks (figs. 2 and 3), spines, tentacles (fig. 7), or a combination of these structures. The scolex may also be unadorned or simple, or it may be absent altogether. The segments have their origin in the neck region or the posterior portion of the scolex if a neck is absent. As the segments progress posteriorly, they become sexually mature (fig. 4), and, after fertilization, become gravid (fig. 6). Most tapeworms are hermaphroditic, and, as a result, the mature segments contain both male and female reproductive organs (e.g., testes, vas efferens, vagina, ovary, etc.). The gravid segments are positioned in the posterior portion of the strobila, and are either characterized with the uterus distended with eggs, or with eggs enclosed within specialized capsules. Code: A Bothrium (pl., bothria). B Rostellum with hooks. C Sucker. D Accessory sucker. E Trilocular bothridium (pl., bothridia). F Excretory canal. G Testis. H Vas efferens. I Cirrus sac. J Vagina. K Ovary. L Vitellarium. M Tentacle. N Tentacle sheath. O Bulb. P Uterus with eggs. Q External seminal vesicle.