Cestoda is the class of parasitic flatworms commonly known as tapeworms. They live in the digestive tract of vertebrates as adults and often in the bodies of various animals as juveniles. 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. Symptoms may include upper abdominal discomfort, diarrhea, and loss of appetite. Tapeworms can grow 15 to 30 feet (10 meters) in length. The largest tapeworms grow up to 59 feet (18 meters). Most tapeworms enter humans through infected food. Tapeworms harm their host by stealing vital nutrients, causing malnutrition and, intestinal blockages.