Tapeworm. Coloured scanning electron micrograph (SEM) of a tapeworm (Taenia pisiformis), showing the hooks (upper right) 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. Magnification: x100, when printed 10 centimetres wide.