However, the above-mentioned "rule" appears to be too general 

 and insufficiently precise. Already from Table 7 we see that Urocleidus 

 dispar is encountered almost in equal quantities both among Eupomotis 

 gibbosus and among Lepomis macrochirus, fishes closely related to each 

 other which are placed by many into one genus Lepomis (actually in the 

 work of Hargis E. gibbosus is placed in the genus Lepomis) , whereas in 

 the relatively distant species Chaenobryttus it is encountered in very 

 small numbers. Close relations are apparent also among U. ferox, the 

 main host of which nevertheless is Lepomis macrochirus . Similar cases 

 are not rare where there are two "basic" hosts for a determined species 

 of monogenetic trematode. This we see in Dactylogyrus vastator Nybelin 

 which occurs almost in the same percentage on the Carp (Cyprinus carpio, 

 nobis) and Karas (Carassius sp. nobis), in D. tuba Linstow on the Yaz 

 (Leucisus idus, nobis) and Zhereh (Aspius aspius, nobis), in Tetraonchus 

 monenteron (Wagener) parasitizing Esox lucius L. and_E. reicherti Dyb. , 

 etc. In addition to that, in separate cases we encounter a large number 

 of "basic" hosts but it is not possible to say which one of them is "the p. 246 



most basic. " Herrings of the genus Alosa for Mazocraes alosae could serve 

 as a sample of such "basic" hosts. Thus, in the Caspian Sea this 

 monogenetic trematode is encountered on A. kessleri (Grimm) in 80-100%, 

 on A. brashnikowi (Borodin) in 70-100%, on A. saposhnikovi (Grimm) in 

 100%, and on A. caspia (Eichwald) in 40-92% which is approximately the 

 same frequency on all four types. As a rule the presence of two or more 

 basic hosts shows that the latter are considerably closer to each other than 

 to other hosts. For instance, Diclybothrium armatum Leuckart infects 

 all the representatives of the genus Acipenser of the Delta of the Volga in 

 almost the same way and is seldom encountered on Huso huso (L. ). We 

 did not find this species even once on the last host (Dogiel and Bychowsky, 

 1938) , and Ivanov and Muregin (1936) found it only once. As an example 

 of such relations, Ancyrocephalus paradoxum Creplin parasitizes, in more or 

 less the same high percentage, on 



Psudak (Lucioperca sp. nobis) and little Berch [ Lucioperca volgensis 

 (Gmelin; ] which is encountered relatively seldom on the Okoun ( Perca sp. 

 nobis ) and is never encountered on the marine Psudak (L^ marina Cuv. ). 

 Without dealing with this question any further we can accept wholly that 

 with the occurrence of the species of monogenetic trematodes on a number 

 of species of fishes of one or several genera, families, or even orders.it 

 is encountered on one or several closely related ones in a larger percentage 

 and for the most part in greater numbers than on others which are far re- 

 moved phylogenetically: On the average, the percentage of infection of the 

 latter decreases in proportion to the separation from the basic host. It 

 is understandable that this normality depends to a great extent on con- 

 crete geographical and ecological conditions in which the given species is 

 located. Thus, the degree of infection of the Okoun (Perca sp. , nobis ) 

 by Ancyrocephalus paradoxus Creplin which was discussed previously de- 

 pends upon the conditions of the body of water in which they live and 

 particularly whether or not the Psudak (L. sp. , nobis ) are present there 

 and what is the numerical population of the Okoun. 



276 



