The genus Isancistrum is isolated by Fuhrmann (Fuhrmann, 

 1928) into a special subfamily- -Isancistrinae, which is accepted in the 

 subsequent resunnes of Price (Price, 1937c), Sproston (Sproston, 1946) 

 Dawes (Dawes, 1946) and others. Nevertheless we can consider, as was 

 already indicated in our work in 1937, that such a separation is not in 

 order because the absence of middle hooks cannot be considered as a 

 characteristic differentiating the subfamily. This, in particular, concerns 

 Gyrodactylidea in which, in the family Polystomatidae we see a considerable 

 fluctuation in the number of naiddle hooks which at best is of generic 

 importance taxonomically. The relation of Isancistrum and three other genera 

 of the family is not clear. It is more probable that this genus is a derivative 

 of Gyrodactylus which changed to parasitizing cephalopods and lost the 

 middle hooks secondarily. As confirmation for this can serve the extreme 

 rarity of the finding of Isancistrum and the absence of the finding of other 

 representatives of Gyrodactylidae on cephalopods in spite of special 

 searches which were conducted by us in 1946 in the region of southern 

 Sakhalin. ^ 



1 



A certain number of squids examined by us in the Pacific Ocean in 

 1955 during the 22nd cruise of the expeditionary vessel "Vityaz" also 

 were uninfected. 



The interrelations of Gyrodactylus, Gy rodactyloides , and 

 Paragyrodactylus are more clearly determined. Without any doubt, both 

 last genera descended from Gyrodactylus - type ancestors independently 

 of each other, and in both cases morphological complications proceed 

 along the lines of acquisition of peculiarities (supplementary chitinous 

 parts) of the attaching disc serving for the preservation of its constantly 

 unfolded middle part. This peculiarity is undoubtedly a progressive one which 

 permits a tighter and easier attachment of the worms to the body of their 

 hosts and has an especially important significance in the cross -infection 



(transfer from one host to another, nobis) of the hosts (let us remember 

 that the infection of the hosts takes place without the larva- -by the adult 

 worms). 



2. Suborder Polyopisthocotylinea (Odhner, 1912) 

 Bychowsky, 1937 



Gyrodactylidea of middle and large sizes; egg -laying. The 

 larvae have 16 edge hooks and often inceptions of the middle ones. 

 Sometimes the larvae bear one pair of suckers in addition (Sphyranuridae). 

 The adult worms have an attaching disc equipped with 2 or 6 muscular 

 suckers and chitinous armature consisting of 16 edge and 1-2 pairs of 

 middle hooks (which can be absent). The connecting apparatus of the p. 400 



478 



