Animated scanning electron micrograph (SEM) of a tapeworm (Taenia pisiformis), pulling back from its head to show its long thin body. Its head bears a ring of hooks that it uses to cling to its host. Tapeworms are parasitic flatworms that live in the digestive tract of their vertebrate host. T. pisiformis is one of the most common parasites of the dog (the other being Dipylidium caninum). It is found worldwide.