Trematodes belonging to the blood-inhabiting group have long attracted the attention of parasitologists and medical men, especially those trematodos belonging to the family Schistosomidae because throw widely distributed species, Schistosoma Japonicum, S. haematobium, and S. mansoni, are parasitic in man, and in endemic regions are the cause of important human diseases. These schistosomes, with a few exception, have undergone more detailed research than any other trematodes. Since the discovery of the first digenetic trematode exhibiting sexual dimorphism by Rudolphi (1819), which was named Distoma canaliculatum, and the descriptions of the other blood flukes up until the present time, a vast amount of material has been put forth in publications having a more or less limited circulation. The writer has, therefore, attempted to assemble descriptions of all the genera and species, hoping that this will be an aid in the identification of these trematodes. Unfortunately many of the descriptions are inadequate as they are based in many instances upon a limited number of specimens. In compiling these data the writer was struck by the possibility of further using them to investigate the genetic and evolutionary relationships of the blood-inhabiting trematodes. Looss (1899) noticed a similarity in the structure, the dermo-muscular sac of the suckers, and in the details of the digestive tract in those blood flukes that had been found. He concluded that their adaptations to existence were similar, as both had a constant environment and a common food supply. Odhner, (1912) through a long study, concluded that the blood flukes were genetically related. The discovery of the life cycles of Sanguinicola and Schistosoma japonicum added further evidence in support of this hypothesis.