However, there is a more striking example of analogous changes 

 of the middle hooks, not within the limits of one family but among the repre- 

 sentatives of various subclasses. Thus, in a number of cases in the middle 

 hooks there appears a special wing- or spur-shaped outgrowth along their 

 edges somewhat above the point which serves apparently as a supplementary 

 adaptation for more firm attachmient of the hook in its penetration into the 

 tissues of the host. This characteristic, peculiar to one species of Dactylo- 

 gyrus (D. pterocleidus Gussew, 1955) is encountered among representatives 

 of the genus Urocleidus { = Pterocleidus of authors) (Mueller, 1936; Mizelle, 

 1938), and in one more as yet undescribed genus close to Anchylodiscoides 

 which was discovered by us on an Indian fish (Pseudotropius garua H. B. ). 



Fig. 311. Middle hooks of certain Dactylogyridae. A-- Dactylogyrus 

 varicorhini Bychowsky from the gills of Varicorhinus buhsei (Kessl. ) from 

 the rivers of Iran; B- -Dactylogyrus markewitschi Gussew,from the gills of 

 Saurogobio dabryi Bl. from Lake Hanka; C - -Falciunguis parabramis 

 Achmerow, from the gills of Parabramis pekinensis (Bas) from the Amur 

 River. 



However, in addition to these cases which exist among the representatives 

 of the same family, Dactylogyridae, just as the preceding ones, the sanne 

 peculiarity in the structure of the middle hooks also exists in a number of 

 (all?) species of the genus Pricea Chauhan (see Chauhan, 1955 and also 

 Figure 312), i.e., among representatives of Gastrocotylidae far removed 

 from Dactylogyridae as is clearly apparent from the phylogenetic diagram. 

 The examples cited indicate that a particular chitinous structure very often 

 (but not always) can show connpletely identical changes in the adaptation to 

 analogous conditions of existence independently of the degree of phylogenetic 

 proximity of the hosts. It seems to us that here takes place not simply a 

 converging similarity of homologous structures, but the manifestation of 

 those internal structural possibilities about which we have spoken some- 

 what earlier and to which we draw attention only because of the singularity p. 466 

 (uncommonness) of their distribution and their relative rarity of appearance. 

 The explanation of the appearance of such peculiarities by summarizing in 

 (or, conjecturing that they are? , nobis ) unoriented variations seems 



to us to be a longer stretch than the attenript to understand it as a verification 

 of the internal structural peculiarities of a given morphological forna. 



558 



