291] NORTH AMERICAN MONOSTOMES 73 



to this author Cyclocoelum mutabile and Typhlocoelum flavum are very 

 closely related and because of different stimuli in their respective habitats, 

 viz.: alveolar spaces, abdominal cavity and liver, occasionally the intestine 

 for Cyclocoelum, and trachea and bronchi for Typhlocoelum, the acetabu- 

 lum of Cyclocoelum has been lost while the oral sucker of Typhlocoelum 

 has atrophied. 



In his earlier work (1902) Cohn described Monostomum oculobium 

 collected from Vanellus melanogastrus as having neither oral nor ventral 

 suckers and relates it to Cyclocoelum mutabile. Fuhrmann (1904) describes 

 a species, Bothriogaster variolaris collected from the intestine of Rostrhamus 

 sociabilis, a South American Falconid, which from his figures and descrip- 

 tion appears to be very similar to Monostomum oculobium of Cohn. He 

 states that in regard to the intestinal crura, absence of the oral sucker 

 and the presence of only a pharynx it is like Cyclocoelum mutabile. The 

 position of the genital glands is not the same but on the other hand is like 

 those of Monostomum oculobium of Cohn. But it differs from Monostomum 

 oculobium in that a ventral sucker is present which Fuhrmann believes 

 Cohn had overlooked in his species. Fuhrmann would place Bothriogaster 

 variolaris in the sub-family Syncoelinae of the Fasciolidae. 



Odhner (1907) supports the view of Cohn and cites a number of in- 

 stances to demonstrate it. Chief among these are, first, his genus Apora- 

 cotyle which is a blood parasite. He says that this suckerless form has its 

 nearest relative in the distome Hapalotrema constrictum (Leared), a blood 

 parasite of the sea turtle. Second, those inhabiting the airsacs have 

 developed the hold-fast organs to the least degree. This includes the 

 Cyclocoelidae in which Odhner says all species are without a ventral 

 sucker except for the recent discovery by Cohn of an entirely rudimentary 

 acetabulum in Typhlocoelum flavum. He adds that in the Holostomes 

 and Hemistomes ventral suckers have been greatly reduced and in some 

 instances have disappeared entirely. In further support of this view he 

 cites the reduction of the sucker in the male Bilharzia with the complete 

 loss of this organ in Bilharzia kovalewski, and notes also the Echinostome 

 like genus Pegosomum which inhabits the gall duct and has lost entirely 

 the oral sucker. Odhner believes that the Monostomata will be finally 

 split up and appended to other trematode groups, i.e., to the Distomes, 

 Amphistomes, Holostomes and perhaps others. 



In the light of the foregoing one may well ask, what is a Monostome? 

 The question has been aptly raised as to what characters or combinations 

 of characters afford a reliable and accurate basis for the natural classifica- 

 tion of the trematodes. On the basis of the examples given above the 

 presence or absence of an acetabulum can be considered of relatively 

 little importance and if such be the case other factors must be looked for 

 in an attempt to establish a natural system of classification. 



