North American Monostomes Primarily From Fresh Water Hosts

carefully and consequently have been transferred to other genera. Out of this has arisen the problem of the origin of the Monostomata. Accumulative evidence has lead to the belief that these forms are directly related to various other groups. This evidence is presented below. Certain investigators of recent time have come to consider the trema- todes of polyphyletic origin. According to Faust (1918) these conclusions are the result of "lack of study and consequent inability to recognize the fundamental resemblance of the genital, excretory and nervous systems." The first to suggest relationship between the Monostomata and the Distomata was Monticelli (1893:149-150) when he called attention to the similarity of Kollikeria and Didymozoon. More recently Ariola (1906) reinforced this opinion by grouping Monostoma fillicolle Rud. and Distoma okeni Kolliker together on the basis of their anatomical similarity even though Monostoma fillicolle does not possess an acetabulum. MacCallum and MacCallum (1916) on the basis of anatomical similarity grouped together the two genera Kollikeria and Nematobothrium altho Kollikeria shows in many cases well developed acetabula while Nematobothrium is in that respect typically monostomatous. Cohn (1904) in his study of Monostomum flavum Mehlis, worked over by Stossich (1902) and placed in the new genus Typhlocoelum, found a well developed but small ventral acetabulum which he figures in sagittal sections. This species on the one...