families of fishes and a small number on families not related to the first 

 ones or to each other; b) or the occurrence on two unrelated groups of 

 families and, within the limits of each group, the families which compose 

 it are related to each other. 



Only 4 cases form exceptions: the occurrence of the genera 

 Gyrodactylus, Heterobothrium, Metahaliotrema, and Acolpenteron on 

 families of fishes unrelated to each other. However, as was previously 

 indicated, the monolithic nature of Heterobothrium is not clear and the 

 genus Metahaliotrema is poorly known to us and consequently, only 2% of 

 the total number of monogenetic trematodes appear to be authentic 

 exceptions. However, we shall attempt to show that the case of Gyrodactylus 

 is not a simple exception to the general rule. 



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