Diplozoon Paradoxum

a hermaphroditic flatworm of the class Monogenoidea that parasitizes the gills of freshwater cypri-nid fishes. Young individuals seek each other out and fuse together permanently at the middle of their bodies, forming a sexually mature individual shaped like an X. Almost all internal systems (nervous, excretory, digestive) except the reproductive system maintain their independence. The male excretory ducts of one individual fuse with the female sexual ducts of the other, and vice versa. In this way constant cross-fertilization is achieved.