Seal tapeworm. Coloured Scanning Electron Micrograph (SEM) of a tapeworm (Diphyllobothrium) from a seal. Diphyllobothrium species are the largest tapeworms that can infect people, and can grow up to 30 feet long and infect a large number of mammalian species including pinnipeds (seals). Infection is through consumption of raw or undercooked fish. While most infections are asymptomatic, complications include intestinal obstruction and gall bladder disease caused by migration of proglottids. Each side of the scolex has a slit-like groove, which is a bothrium for attachment to the intestine. Magnification: x30 when printed at 10 centimetres wide.