Cestoda | Research & Encyclopedia Articles

The class Cestoda consists of long, flat, ribbonlike worms that are commonly called tapeworms. Tapeworms are obligatory parasites, ones that cannot survive independent of a host, that live in the intestines of vertebrate hosts. They form an extremely varied group, and nearly every vertebrate species is associated with a different parasitic cestode. Most cestodes make use of one or more intermediate hosts to bring them into the body of the ultimate host. Some cestodes can achieve impressive lengths—worms of up to 15 meters (50 feet) have been observed.

All tapeworms share a body plan. At the front end is a head region called the scolex. The scolex maintains a hold on the host's digestive tract and has many suckers and hooks for this purpose. The scolex also contains the tape-worm's sense organs, which consist primarily of cells sensitive to touch and chemical stimuli, as well as the modest concentration of nervous tissue that makes up the tapeworm brain.

The scolex is followed by a short neck region and a trunk, which is divided into a series of segments known as proglottids. New proglottids are produced in the neck region. As these form, older proglottids are pushed back toward the rear of the animal. The proglottids house the reproductive organs, which mature gradually as proglottids move to the back.

Tapeworms are hermaphroditic, so that each proglottid includes both male and female gonads and generates both sperm and eggs. A tapeworm can reproduce sexually, either through self-fertilization or cross-fertilization with another tapeworm, or asexually, by breaking off proglottid segments at the end of the trunk. These reproductive traits are admirably adapted to reproduction in an environment (in the body of a host) in which worms are not guaranteed to encounter individuals of the same species.

Proglottids and fertilized eggs exit the host's digestive tract along with the host's excrement. In most tapeworm species, eggs or proglottids are first ingested, or taken in, by an intermediate host, often an arthropod or a different vertebrate species. The cestode may develop into a larval form or may become temporarily dormant within the intermediate host. The ultimate host becomes infested with the cestode when it consumes an infested intermediate host.

Because of the cestodes' parasitic lifestyle, certain organ systems are unnecessary. The most obvious of these is the digestive tract, which is absent from the group. Because cestodes live in an environment that is not only rich in nutrients, but one in which the nutrients are already well processed, further digestion is unnecessary. Instead, food absorption occurs over the entire surface of the cestode body, in an ectodermal, or skin, layer known as the integument. The integument is covered with tiny projections called mitotrichia, which increase the surface area available for absorption.

Cestodes are divided into two subclasses, Cestodaria and Eucestoda. Cestodaria is a small subclass of relatively small tapeworms that are parasites to elasmobranch fishes (sharks, rays, and chimeras). The trunks of cestodarians are not segmented into proglottids. The rear of the body includes a small sucker. Eucestoda is a much more diverse group, and includes all other cestodes. Eucestodes are characterized by the presence of proglottids.

Phylogenetic Relationships of Major Groups.

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton and Co., 1996.

Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: Wm. C. Brown, 1994.

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