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. The largest tapeworms can be 20 m or longer.
Roger Wilco encountered a Tapeworm inside Stellar Santiago's body. Although initially hostile, Roger gives it a Timed-Release Time Pill and he was then able to use it to ride Stellar's digestive tract.
For more information on Tapeworm's, see Wikipedia's entry on tapeworms.