Before speaking about the distribution of Monogenoidea, one 

 must point to the considerable difficulties connected with the correct 

 determination of the host of the parasites --that is, on the first order-- 

 fishes. On this, in considerable measure, depends the correctness of 

 the subsequent conclusions. Unfortunately, in this connection one must 

 consider the data of a number of authors with great reservation. Equally- 

 sad, and what also impedes the work, is the fact that in many works, 

 especially of the past century, the same fishes are cited under different 

 species names. This condition is worsened by the confusion of synonyms 

 of certain groups of fishes which even an ichthyologist finds difficult to 

 untangle. 



The system of fishes and their division into orders were 

 takeei by us from the work of L. C. Berg (1940) but with two changes. 

 Thus, L. C. Berg takes tunas (Thunnidae) as a special order-- 

 Thunniformes; we on the other hand, in conformity with the opinion of the 

 majority of contemporary ichthyologists, refer this order to the perch- 

 bass family (Perciformes, nobis) placing it close to Cybiidae (in the 

 understanding of L. B. Berg). L. C. Berg also speaks about the proximity 

 of tuna with the latter in his work. 



In addition to that, instead of the decimal system which divides 

 contemporary Selachii into 6 orders as is accepted by L. C. Berg, we 

 consider it more convenient to recognize all Selachii as one order-- 

 Selachiiformes ,in conformity with the opinion of G. V. Nikolsky (1954). 



This work is considerably hampered by the absence of well- p^ 221 

 developed broad, but detailed phylogenetic schemes for fishes. Further, 

 in addition to the system of L. C. Berg, which bears a rather formal 

 character according to our conviction, we have utilized a number of 

 papers which will be referred to in appropriate places. For the time 

 being, we shall only note that the scheme of interrelations of the bony 

 fishes of Gregory (Gregory, 1951) gave us very many useful indications 

 for further systematizations and apparently has much that is logical in 

 its foundations. Parasitological data connected with this will be ex- 

 pressed somewhat later. We consciously avoid speaking now about the 

 general theoretical questions connected with the problem of occurrence 

 and specificity of parasitic animals, considering that for their useful 

 discussion it is indispensable to express the factual material first. 



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