In biology, Cestoda is the class of parasitic flatworms, called cestodes or tapeworms, that live in the digestive tract of vertebrates as adults and often in the bodies of various animals as juveniles. In a tapeworm infection, adult worms 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. Tapeworms harm their host by stealing vital nutrients, causing malnutrition and if left untreated can cause intestinal blockages. 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.