is based on the possibility of a sharp change of conditions of Jhe host and 

 subsequently of its parasite, of the survival of the first during the death of 

 the second. However, all our data indicate that usually Monogenoidea are 

 not less enduring than their hosts and perish either at the same time with 

 them or even, as is often the case (in artificial conditions), after them (see 

 experinnents of Shulmann, page 79). 



However, let us return to Dactylogyridea. The morphological 

 analysis of this order forces us to divide it into 2 sufficiently distinct 

 separate suborders - -Dactylogyrinea and Monopisthocotylinea. The analysis p. 455 

 of occurrence of the representatives of these suborders shows that the first 

 are peculiar only to Teleostei, whereas the second parasitize both these 

 fishes and selachians. With this, the analysis which was cited in the pre- 

 ceding chapters gives us reason to believe that the basic part of the families 

 of Monopisthocotylinea are encountered on Selachii indicates them to be 

 their initial parasites and apparently very ancient ones. Hence, the suppo- 

 sition that the second suborder and not the first is more ancient in the time 

 of separation becomes quite probable, although, repeating what has been 

 said about it, morphologically the first is considerably closer to the 

 supposedly ancestral forms. It will be better, however, to express the 

 considerations about the possible time of separation of both suborders after 

 the examination of the connections of separate families within the limits of 

 each suborder. 



Dactylogyrinea represents a more compact group than Monopistho- 

 cotylinea. Two families which enter the composition of the first suborder, 

 namely Dactylogyridae and Diplectanidae, are characterized by the presence 

 of 14 edge hooks of the attaching disc, while two others --Protogyrodactylidae 

 and Calceostomatidae, have only 12 edge hooks. However, as has already 

 been mentioned before (see page 360 ), there is strong basis to doubt the 

 correctness of the presence of only the 12 hooks on Protogyrodactylidae, it 

 is more possible that they also have 14 hooks just like the Dactylogyridae^ 

 with which they are very close and with which they are closely linked 

 genetically, independently of the solution of the question about the quantity 

 of edge hooks. There are all necessary indications to suppose that this 

 family descends directly from Dactylogyridae and even definitely from 

 Ancyrocephalinae and thus it is relatively young. Taking into consideration 

 that Protogyrodactylidae are encountered only in Australia and only in fresh- 

 water Serranidae one can consider that they separated no earlier than the 

 Eocene period. All the other families of the suborder undoubtedly are of 

 more ancient origin and thus Protogyrodactylidae is historically one of the 

 youngest families of Monogenoidea. 1 



1 



This once more shows how unfortunate it was to name the group as was 

 done by Johnston and Tiegs. 



546 



