72 ILLINOIS BIOLOGICAL MONOGRAPHS [290 



THE POLYPHYLETIC ORIGIN OF THE MONOSTOMES 



From the earliest records of the Monostomata up to the present time 

 this group of parasites has served for a dumping ground for inaccurately 

 studied species in which the acetabulum has been wrongly interpreted or 

 overlooked entirely. Many species have since been studied more 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 var- 

 ious 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 Monostonia fillicolle Rud. and Distoma 

 okeni Kolliker together on the basis of their anatomical similarity even 

 though Monostonia 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 hand is apparently very closely related 

 to the genus Cyclocoelum and was placed by Stossich in the same sub- 

 family, Cyclocoelinae. On the other hand Cohn would transfer this to 

 the Fasciolidae because of the presence of the ventral acetabulum which 

 he says is diminished and in other instances often lost because of the shut-in 

 habitat under which these worms live. 



He adds as was stated previously the observation of a rudimentary 

 mouth sucker in Cyclocoelum mutabile and in one other species of this 

 group. Here he states that Cyclocoelum mutabile does not lack a primary 

 sucker in many cases and like the Cestodarian Amphilina has lost hold-fast 

 organs because of the lack of need for such organs in the cavities of the 

 body of the host in which habitat these worms are wont to live. According 



