Phylogeny of the Polystomatidae (Platyhelminthes, Monogenea), with particular reference to Polystoma integerrimum

Polystome phylogeny is examined, with emphasis on the dimorphism of the frog parasite Polystoma integerrimum, which exists in a fully differentiated and a neotenic form, and the evolutionary development of exclusively neotenic genera. Protopolystoma, which infects the aquatic toad Xenopus, has essentially the same morphology as the neotenic (branchial) adult of P. integerrimum and is interpreted as a neotenic genus; however, it inhabits the bladder of its host, the infection site of the normal adult of the dimorphic species. The sphyranurid Sphyranura, ectoparasitic on the external gills of the mud puppy Necturus, resembles the two-sucker larva of P. integerrimum in possessing a single pair of haptorial suckers in place of the 6 suckers of adult polystomes, and is probably a neotenic parasite associated with a neotenic host. The neotenic animals in general are parasites of aquatic hosts, and the uterus is lost or reduced in these genera; by contrast, uterine function is greatly enhanced among polystomes infecting amphibians best adapted to terrestrial life.