We report on a comparative scanning electron microscopical study of trophi morphology of the two species making up the Class Seisonidea of Phylum Rotifera. The trophi of the two are generally similar, with morphological differences being interpreted as the result of adaptations to different food sources: the trophi of Seison nebaliae are adapted to particle-feeding, that of Seison annulatus probably functions by piercing the integument of the host using the fulcrum tip, and subsequently sucking out haemolymph. This difference in feeding ecology is believed to contribute to the co-occurrence of both species on the same host. A new hypothesis on the evolution of rotifer trophi is proposed.