Rohde, K: Comparative ultrastructural study of the posterior suckers of four species of symbiotic Platyhelminthes. Temnocephala sp. (Temnocephalida), Udonella caligorum (Udonellidea), Anoplodiscus cirrusspiralis (Monogenea: Monopisthocotylea), and Philophthalmus sp. (Trematoda: Digenea)