In biology, Cestoda is the class of parasitic flatworms, called tapeworms, that live in the digestive tracts of vertebrates as adults and often in the bodies of various animals as juveniles. In a tapeworm infection, adults absorb food predigested by the host, so the worms have no need for a digestive tract or a mouth. Large tapeworms are made almost entirely of reproductive structures with a small "head" for attachment. Symptoms vary widely, depending on the species causing the infection. Symptoms may include upper abdominal discomfort, diarrhea, and loss of appetite. However, infestations are usually asymptomatic. Worm segments or eggs may be found in the stool of an infected person.

The largest tapeworms can be 80 feet or longer. Tapeworms harm their host by stealing vital nutrients, causing malnutrition and if left untreated can cause intestinal blockages.

Adult tapeworms share a basic body plan. All have a scolex, sometimes colloquially referred to as the "head," a "neck," and one or more proglottids, which are sometimes called "segments," and which are the source of the name "tapeworm," because they look like a strip of tape. All cestodes have a nerve ring in the scolex with lateral trunks passing through the rest of the body.

The Scolex or "head" of the worm attaches to the intestine of the definitive host. In some groups, the scolex is dominated by bothria, which are sometimes called "sucking grooves," and which function like suction cups. Other groups have hooks and suckers that aid in attachment. Cyclophyllid cestodes can be identified by the presence of four suckers on their scolex, though they may have other structures as well.

While the scolex is often the most distinctive part of an adult tapeworm, it is often unavailable in a clinical setting, as it is inside the patient. Thus, identifying eggs and proglottids in feces is important.

The Neck of a tapeworm is a relatively undifferentiated mass of cells that divide to form new proglottid "segments." This is where all growth in an adult tapeworm occurs.

Posterior to the scolex, they have one or more proglottids that hold the reproductive structures. The sum of the proglottids is called a strobila. It is shaped thin like a strip of tape, which is the source of the common name tapeworm. Like some other flatworms, cestodes use flame cells (protonephridia) for excretion, which are located in proglottids.

Mature or gravid proglottids are released from the mature tapeworm and leave the host in its feces.

Because each proglottid can reproduce independently, it has been suggested by some biologists that each should be considered a single organism, and that the tapeworm is actually a colony of proglottids.

Tapeworms can be effectively cured with antiparasitic (antihelmintic) medication once it is detected. Current Centers for Disease Control and Prevention guidelines for treatment is a prescription drug called praziquantel. The medication causes the tapeworm to dissolve within the intestines. Praziquantel is generally well tolerated. Sometimes more than one treatment is necessary.

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