Fish parasite. 19th century artwork of the Diplozoon paradoxum fish gill parasite (upper frame). This unusual flatworm has bilateral symmetry as it is actually two individuals fused together. In the lower frame are detailed views of the haptor, the organ used by the parasite to attach to the fish gill. Artwork of the fish Artwork from 'Micrographic Contributions to the Natural History of Invertebrates' (1832) by the Finnish biologist Alexander von Nordmann.