Cestoda
Cestoda is the name given to a class of parasitic flatworms, of the phylum Platyhelminthes. Informally biologists refer to them as Cestodes. The most notorious species commonly are called
tapeworms, for example
Taenia solium, the "pork tapeworm". Cestodes are all parasitic and their life histories vary, but typically they live in the digestive tracts of vertebrates as adults, and often in the bodies of other species of animals as juveniles. Over a thousand species have been described, and all vertebrate species may be parasitised by at least one species of tapeworm. Humans are subject to parasitism by several species of tapeworms if they eat underprepared meat such as pork, beef, and fish, or if they live in, or eat food prepared in, conditions of poor hygiene.
T. saginata, the beef tapeworm, can grow up to 20 m; the largest species, the whale tapeworm
Polygonoporus giganticus, can grow to over 30 m. On the other hand, species using small vertebrates as hosts tend to be small. For example, vole and lemming tapeworms are only 13–240 mm in length, and those parasitizing shrews only 0.8–60 mm.