CHAPTER VII 



FAUNA OF THE MONOGENOIDEA OF SEPARATE GROUPS 



OF THEIR HOSTS. 



For further analysis of links between the hosts and the mono- p. 300 

 genetic trematodes encountered on them one must examine the occurrence 

 of the latter from a different point of view, namely- -to attempt to examine 

 the nature of the fauna of monogenetic trematodes of definite groups of their 

 hosts in order to determine first their monolithic state or conversely their 

 heterogeneity and then to utilize this for the purpose of refining the phylo- 

 genesis of Monogenoidea. At the same time one must not forget the 

 opposite --the possibility of utilizing the data on Monogenoidea for the 

 correction of contemporary conceptions about the phylogenesis of their 

 hosts. Both can be considered fully possible inasmuch as we have already 

 shown the indubitable phylogenetic significance of the nature of the 

 occurrence of monogenetic trematodes. 



All three classes of fishes with which we have to deal--Elasmo- 

 branchii, Holocephali and Teleostomi are distinctly separated frona each 

 other and the first two, as is known, are encountered from the upper 

 Devonian and the third even from the lower Devonian. Without any doubt 

 the first two classes are much closer to each other thaneitherof them is 

 to Teleostomi. If we look at the fauna of Monogenoidea of these three 

 classes, the first thing that becomes apparent is that both subclasses of 

 monogenetic trematodes are encountered on all three classes of fishes; 

 however, their nature of infection is completely different. Thus, Poly- 

 onchoinea, Dactylogyridea and Tetraonchidea are known from Elasmo- 

 branchii, Holocephali and Teleostomi; whereas, Gyrodactylidae are 

 encountered practically only on the latter. Likewise among Oligonchoinea, 

 Mazocraeidae are encountered only on Teleostomi, Chimaericolidea 

 exclusively on Holocephali--whereas Diclybothriidae are encountered on 

 representatives of all three classes. However, the general data repro- 

 duced are very coarse and do not give the correct idea about the nature of 

 the fauna of Monogenoidea on each order. 



So far only five species of Monogenoidea are known on Holocephali. 

 Two of them pertain to the order of Chimaericolidea and make up its entire 

 mass. Thus, this order is fully specific for Holocephali and characterizes 

 it. As regards the three other species, one of them, Callicotyle affinis 

 Th. Scott (order Dactylogyridea), is encountered both on Chimaera 

 monstrosa L. and on Raja fuUonica L. (see page 228). Beyond any doubt, 

 in spite of the fact that this species is basically encountered on the first 

 host, it is a parasite which chanced upon Holocephali secondarily and 

 which has a relatively short connection with Holocephali as far as time is 

 concerned. The second species --Gyrodactylus moandrica Mendival- 

 Herrera, is undoubtedly secondary, if not a fortuitous parasite of the 



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