Dr. Murai Éva szerk.: Parasitologia Hungarica 16. (Budapest, 1983)
similar to the pattern of amphistomes of amphibians than that of any other digeneans, thus their relegation into this family is justifiable. The existance of a species of amphistomes ( P. cornu, characterized by having one testis) among the parasites of fishes (characterized by having two testes) can be explained, from the phylogenetical point of view, by the assumption that in the ancient stocks of amphistomes (in the Silurian and Devonian times when fishes evolved) such forms emerged whose features were more characteristic for amphistomes of the subsequent vertebrates' (amphibians) parasites than those of their own group of hosts (fishes). Hence, the main feature (single testis) of this species, as a fish amphistome, is an apomorphous one and as compared with that of amphibian-inhabitant amphistomes, it is plesiomorphous feature. In case of D. trifoliata the situation is somewhat different. This species is a parasite of turtles but morphologically it belongs to the family Diplodiscidae. It probably differentiated at the periphery of the ancient stock of the ProgonimodiscusCatadiscusPseudodiplodiscus line and adapted to the reptilian hosts. This adaptation resulted in the feature characteristic of reptilian-inhabitant amphistomes in its acetabulum: the presence of muscular pad, which is an apomorphous trait among the diplodiscids. The cases of these species in question indicate that the taxonomic position (phenetic similarity) does not coincide rigidly with the scope of the definitive hosts. The phylogenetical situation of the species of the genus Catadiscus, found in snakes has not been cleared up yet. It is questionable whether they are real parasites of these hosts or that they are the case of the secondary transmission which is called by BOZKOV (1976) as postcycle parasitism. Nevertheless, the latter assumption seems to be supported by the fact that these flukes were recovered in the intestine and not in the rectum which is the usual their habitat in amphibians. Furthermore, there are two more amphistomes ( Chiorchis /now Stunkardia / burti and Alla s- sostomoides spp. *)of which the former is a parasite solely of amphibians and the latter is that of both amphibians and reptiles with the same frequency. Although S. burti is an amphibianinhabitant parasite morphologically (general appearance, structure of pharynx, form and arrangement of testes and vitellaria), however, it is more similar to those of reptiles. This species, among the amphistomes of amphibians, represents apomorphous states and its phylogenetic importance is the same as that of P. cornu among amphistomes of fishes. These findings show that there can be found at least one species, in the stocks of the amphistomes with definitive pattern, which is more similar to the pattern of amphistomes of the subsequent host-group than that of any other species of amphistomes of the same group of hosts. Among parasitologists it is well known that the name of a given parasite bears certain information on its host and geographical distribution. The real background of it is the assumption that the parasites have evolved in space and time in co-evolution with their hosts. Thus, it can be said that parasites are characteristic of hosts and, at the same time, hosts harbouring parasites are characteristic of their own in a certain area. Such an interdependence indicates the highly correlated host-parasite phylogenies and geographical distribution (BROOKS, 1979). As the parasite of today's vertebrates had evolved in the ancestons of the present-day vertebrates, as a result of a long-time parallel evolution, their adaptation to a certain host-group predestinates that host-parasite coevolution demonstrates a marked degree of concordance among the patterns of parasite, host and geographical relationship (BROOKS, 1980). Species of this genus show affinity to the reptilian-inhabitant amphistomes on the histomorphological structure of the pharynx and acetabulum. The occurrence of these species in frogs is rather the consequence of the broad ecological valency than that of the tendency of colonization of lower vertebrates by amphistomes parasitizing higher ones.
