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The random amplified polymorphic DNA polymerase chain reaction (RAPD-PCR) technique was applied to two closely-related echinostome species, Echinostoma trivolvis and E. caproni, demonstrate interspecific polymorphisms of genomic DNA. Band patterns generated using five individual primers showed that these two echinostomes were genetically distinct, although they share genomic DNA to some extent.

An isoenzymatic analysis using thin-layer agarose gel isoelectrofocusing on laboratory strains of Echinostoma trivolvis and Echinostoma caproni adults showed characteristic monomorphic phenotypes for phosphoglucomutase and glucose phosphate isomerase. The fixed allelic variation observed between these 2 taxa is consistent with their current classification as distinct species.

Ten species of the trematode family Heterophyidae (Metagonimus yokogawai, M. takahashii, Heterophyes nocens, H. heterophyes, H. dispar, Heterophyopsis continua, Pygidiopsis summa, Stellantchasmus falcatus, Centrocestus armatus and Stictodora fuscatum), and 3 species of the family Echinostomatidae (Echinostoma hortense, E. cinetorchis and Echinochasmus japonicus) have been found to infect humans in Korea. Biological and epidemiological studies on the above species have shown that Metagonimus, heterophyids and echinostomes are all prevalent indigenously in Korea except H. heterophyes and H. dispar, of which the human cases were infected when they came to the Middle East. Various kinds of fish have been proven to be the source of human infection with these flukes. For example, fresh water fish such as the sweetfish, carp, loach, etc., were found to carry the metacercarial stage of M. yokogawai, M. takahashii, C. armatus, E. hortense, E. cinetorchis and/or E. japonicus. Brackish water fish such as the mullet, perch and goby, were verified to be the second intermediate host of H. nocens, H. continua, P. summa, S. falcatus and/or S. fuscatum. Among the intestinal flukes, M. yokogawai is the most prevalent species in Korea.

With reference to a recent taxonomic revision the species of the echinostomes used in key studies on the population regulation in infections with 37-collar-spined Echinostoma species in experimental rodent hosts were reconsidered. This was considered essential to prevent taxonomic problems blocking further fruitful progress within this field of experimental parasitology.

The closely related Echinostoma lindoense, E. audyi and E. revolutum can be differentiated by morphological characteristics of their adults and cercariae. We have found E. lindoense and E. audyi in Southeast and Southwest Asia and Central Europe and the former species also in South America. However, using the morphological characteristics described by Beaver (1937) for E. revolutum which is assumed to be cosmopolitan, we did not find this species in these regions.

The life-cycles of Echinostoma revolutum and E. echinatum were carried out by using cercariae emitted by naturally infected Molluscs. E. revolutum strain was issued from Lymnaea auricularia, E. echinatum from Lymnaea truncatula and Planorbis planorbis, Planorbarius corneus produced an Echinostome which may possibly be E. echinatum or another species E. sp. Metacercarial stages and adults were obtained from laboratory experimental hosts. None of these adult Echinostomes displayed well-defined morphological differences: nevertheless their respective larval stages exhibited discrepancies used for species diagnosis. Cercarial chaetotaxy is given for every batch and compared with that of other species described as E. audyi, E. lindoense and E. caproni; discriminating features are discussed.

In the previous paper of this series, the cercariae from Oncomelania quadrasi (Part 2), from the snails of Thiaridae (Part 3), from the snails of Viviparidae and Pilidae (Part 4), and from Planorbidae (Part 5) were reported continuously. In this paper the crecariae from Lymnaeidae and Bulinidae were dealt with. These studies were all carried on during the period of 1975 and 1976 in Leyte Island, Philippines. From the lymnaeid snails seven species of cercariae, comprising two echinostome cercariae, one fasciolid cercaria, and four xiphidiocercariae were detected. From the bulinid snails, only two xiphidiocercariae were found, both of which were identifiable with those from the lymnaeid snails. These seven species of cercariae are described and illustrated with a tentative name from Cercaria leyteensis no. 37 to Cercaria leyteensis no. 43, in addition with some remarks on their presumptive life histories.

A comparative enzymatic study is carried out on adults of three species of the genus Echinostoma (E. caproni, E. togoensis, E. sp.). Amongst 23 enzymes tested by starch gel electrophoresis, Pgm and Pgi are characterised by different zymogrammes in each species. Agarose and polyacrylamide isoelectrophoresis focusing define isoenzyme genetic structures: Pgm is coded by three loci, Pgi is coded by one locus. Pgm polymorphism allows us to extend this work to other experimental or natural stocks.