Diphyllobothriidean tapeworm. Scanning electron micrograph (SEM). Diphyllobothriidean cestodes have indirect life cycles that require two intermediate hosts before becoming infectious to the definitive host. Mammals (mostly seals and cetaceans) are definitive hosts of the majority of diphyllobothriid. Scolex always unarmed, with a pair of attachment grooves (bothria). Magnification: x23 when printed at 10 centimetres wide.