The development of Gymnophallus australis Szidat, 1962 (Digenea: Gymnophallidae) from the Patagonian coast (Argentina) from metacercaria to adult, with an amended diagnosis of Gymnophallus Odhner, 1905.

Parvatrema australis (Szidat, 1962) Szidat, 1965 was described based on larval stages found in specimens of the mussel Mytilus edulis from the Buenos Aires Province, Argentina. Although Szidat later examined hundreds of mussels, this parasite has never been found again until now. In the present study, larval stages, including germinal sacs, found in mytilids from the Patagonian coast were identified as P. australis. Metacercariae were incubated in vitro at 39 degrees C in physiological solution for 18-20 hours, by which time 80% of the specimens had eggs. P. australis is redescribed, on the basis of infective metacercariae and adults obtained in the laboratory, and is reassigned to Gymnophallus Odhner, 1900, the genus in which it was originally described. The generic diagnosis of Gymnophallus is here amended to include as diagnostic characters the presence or absence of the lateral lips, the form and position of the vitellarium (compact or follicular) and the presence of a pars prostatica (i.e. prostatic cells open into proximal part of the ejaculatory duct). The validity of some characters (i.e. the presence of lateral lips of the oral sucker, the form of the vitellarium and excretory vesicle, the extent of the uterus) as diagnostic at the generic level within the family Gymnophallidae is discussed. It is proposed that the least unambiguous characters that can be used to distinguish gymnophallid genera include the position of the ovary, the presence of a ventral pit and a pars prostatica, and caecal diverticula.