Digenetic trematodes are believed to be originated from the turbellarian ancestors and their parasitic life is assumed to have developed from their ancestors to the phylogenetically related invertebrate hosts. The most primitive digenea, therefore, most have confined their parasitism theoretically to invertebrate hosts until the advent of vertebrates, in the simplest life cycle pattern, i.e. one host cycle. As a matter of fact this simplest pattern is represented by pathogenesis.

Contents: 1. Introduction. 2. Systematic index. 3. Systematic account. 4. Summary. 5. Acknowledgement. 6. Dedication. 7. References.