The tapeworm from Stellar's body.
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 riding the Tapeworm
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.
The scene where Roger is riding the tapeworm is reminiscent of Paul Muad'dib riding the sandworm in the Dune novel and films.