(se′stöid·ē·ə)

(invertebrate zoology) The tapeworms, endoparasites composing a class of the phylum Platyhelminthes.

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A class of the phylum Platyhelminthes commonly referred to as tapeworms. All members are endoparasites, usually in the digestive tract of vertebrates. The class has been subdivided as follows:

Class Cestoidea

     Subclass Cestodaria

          Order:Amphilinidea

                                        Gyrocotylidea

     Subclass Cestoda

          Order:Proteocephaloidea

                                        Tetraphyllidea

                                        Lecanicephaloidea

                                        Trypanorhyncha

                                        Diphyllidea

                                        Pseudophyllidea

                                        Cyclophyllidea

                                        Nippotaeniidea

See separate articles on each group listed.

The tapeworms range from less than 0.04 in. (1 mm) to several feet in length. The class is differentially characterized by the presence of a cuticle rather than a cellular epidermis and by the total absence of a mouth and digestive tract. Food materials are presumed to be absorbed through the external surface. In most species of the class the body is divided into proglottids, each proglottid containing one or two hermaphroditic reproductive systems. The anterior end is usually modified into a holdfast organ, bearing suckers or sucking grooves, and frequently armed with hooks. Early embryonic development occurs in the parental body, usually in a uterus, to a hook-bearing stage, the oncosphere. The oncosphere leaves the parental body through a uterine pore or by liberation of the terminal segment from the main body of the worm. Further development of the worm always occurs within the body of a host, most often an invertebrate, which commonly ingests the larval form. Further larval growth and development of the holdfast may require a second host. Development of the sexual phase from the larva occurs in another host. Growth with strobilation and development of reproductive systems follow, the worm usually staying in the digestive tract. See also Platyhelminthes; Pseudophyllidea.

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(sĕs-toi'dē-ə)
n.

A class of flatworms characterized by lack of an alimentary canal and, in typical forms, by a segmented body with a scolex at one end; adult worms are vertebrate parasites, usually found in the small intestine.