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.
Tapeworms use anaerobic respiration, which breaks down sugars to lactic acid or ethanol. This type of respiration is less efficient than using oxygen for respiration (aerobic respiration), but since the tapeworm has such an easy life, it does not use a lot of energy so anaerobic respiration works for them.