Tapeworms: Cestoda

The body of most tapeworms is flat and much longer than it is wide, so that it looks like a tape or ribbon. The length varies from 0.02 inch (0.6 millimeter) to 98 feet (30 meters), the longest worms being found in sperm whales. Tapeworms are parasites that have no head, mouth, or digestive system. Parasites (PAIR-uh-sites) are animals or plants that live on or in other animals or plants, or hosts, without helping them and usually harming them. Tapeworms have a body covering through which they absorb nutrients from the host's intestine. This covering also protects the worms from the host's immune reactions and digestive acids. Tapeworms are whitish and as internal parasites they live in darkness.

The body of tapeworms has three regions: scolex (SKOH-leks), neck, and strobila (stroh-BYE-luh). The scolex is the head. It has spines, hooks, suckers, tentacles, glands releasing sticky secretions, or a combination of these structures that the worm uses to attach itself to the inner wall of the intestine of the final host, also called the primary host. Suckers are the most common attachment tool. Suckers are usually cup shaped and have powerful muscular walls. The neck is the region of the body just behind the scolex. It is usually short.

The strobila is behind the neck. It consists of a row of segments called proglottids (proh-GLAH-tuhds). The strobila is made up of anywhere from a few to more than one thousand proglottids but usually contains several dozen. Each proglottid starts development at the neck, and proglottids form one by one throughout the life of the tapeworm in the final host. Just behind the neck, the proglottids are short and narrow. When a new proglottid forms at the neck, already formed proglottids are pushed toward the rear, grow, and eventually contain the reproductive organs.

Behind the new proglottids, each strobila contains the following types of proglottids, from front to back: premature proglottids, with the beginnings of reproductive organs; mature proglottids, which contain functioning male and female reproductive organs; postmature proglottids, which contain developing eggs; and gravid (GRA-vuhd) proglottids, which contain ripe eggs. The gravid proglottids at the end of the worm break off and pass into the environment with the host's feces (FEE-seez). A few species of tapeworms have no proglottids.