Class Cestoda

About 5,000 species of tapeworms make up the class Cestoda. Tapeworms can live in the intestines of almost all vertebrates. Humans may harbor any of seven different species. Tapeworms enter their host when the host eats raw or undercooked food containing eggs or larvae. A tapeworm infection may cause digestive problems, weight loss, lack of energy, and anemia, which is a decrease in the number of red cells in the blood.

Structure of Tapeworms

Like flukes, tapeworms are surrounded by a tegument that protects them from their host's defenses. As Figure 34-5 shows, at the anterior end of a tapeworm is a knob-shaped organ called the scolex (SKOH-leks), which has hooks and suckers that enable the worm to attach to its host. A short neck connects the scolex with a long series of body sections called proglottids (proh-GLAHT-idz). As a tapeworm grows, it adds proglottids just behind the neck, pushing the older proglottids toward the rear. A single tapeworm may have 2,000 proglottids and exceed 10 m (33 ft) in length.

The excretory system and nervous system of a tapeworm are similar to those of other flatworms. However, tapeworms lack eyespots and other lightsensitive structures, and they have no mouth, gastrovascular cavity, or other digestive organs. They absorb nutrients directly from the host's digestive tract through their tegument. The tegument is highly folded, which increases the surface area available for absorption.

figure 34-5

A tapeworm grows by adding proglottids behind its scolex. Each proglottid contains both male and female reproductive organs.

A tapeworm grows by adding proglottids behind its scolex. Each proglottid contains both male and female reproductive organs.

figure 34-6

Adult beef tapeworms live in the intestines of humans, their primary hosts. O Proglottids pass out of the primary host in feces, crawl onto vegetation, and release the tapeworm eggs. © A cow, the intermediate host, ingests the eggs when it eats the vegetation. The eggs hatch into larvae that form cysts in the cow's muscles. © When a person eats undercooked beef, the larvae in the beef develop into adult tapeworms in the person's intestine, and the cycle repeats.

Reproduction and Life Cycle of Tapeworms

Nearly all tapeworms are hermaphrodites. You can see in Figure 34-5 that each proglottid contains both male and female reproductive organs, but little else. As the proglottids move to the rear of the tapeworm, they grow, mature, and begin producing eggs. The oldest proglottids are almost completely filled with 100,000 or more eggs. Eggs in one proglottid are usually fertilized by sperm from a different proglottid, either in the same individual or a different individual if the host has more than one tapeworm.

The life cycle of the beef tapeworm, Taenia saginatus, is illustrated in Figure 34-6. Like the blood fluke, the beef tapeworm has two hosts. The primary host is a human. In the human intestine, mature proglottids break off from the adult and are eliminated with the host's feces. If the feces are deposited on the ground, the proglottids crawl out of the feces and onto nearby vegetation. The eggs they release may remain alive for several months before the vegetation is eaten by a cow, the intermediate host. Inside the cow, the eggs develop into larvae that burrow through the cow's intestine and enter the bloodstream. The larvae then make their way to muscle tissue and form cysts, or dormant larvae surrounded by protective coverings. Humans become infected when they eat beef that has not been cooked well enough to kill the worms inside the cysts. Once a cyst enters the human intestine, the cyst wall dissolves and releases the worm. The worm then attaches to the intestinal wall and develops into an adult, beginning the cycle again.

Another tapeworm that infects humans is the pork tapeworm, Taenia solium. Its life cycle is similar to that of the beef tapeworm, except that a pig serves as the intermediate host.

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