Attaching formations not taken separately but as particular 

 systems produce singular converging similarities. As a good example in 

 this connection are the cases of adaptation of the attaching apparatus of 

 certain lowest Monogenoidea for the embracing of the gill filaments of 

 the host, which have a specific length, by the system of middle hooks. This 

 adaptation is especially characteristic for Diplectanidae among which the 3 

 connecting plates are extremely elongated and form an almost inflexible 

 support, fixing 4 middle hooks lying in pairs along the side of the disc at 

 a particular distance from each other. This distance is determined by the 

 width. of the gill filament. Convergently similar structure is acquired by 

 the attaching apparatus of certain Dactylogyridae where the middle part of 

 the system consists not of 3 but only of one primary middle connecting plate 

 and the edges not of 2 but only of one pair of hooks. For instance Dactylogyrus 

 singularis Gussew (Gussew, 1955) has a similar structure. Likewise the 

 strengthening of the system of middle hooks leads to converging similarities 

 among the genera Tetraonchus (Tetraonchidae) and Actinocleidus (Dactylo- 

 gyridap). In the first genus one middle plate is located between the 4 middle 



Fig. 312. Middle hooks of certain Dactylogyridae and Gastrocotylidae. 

 A- -Dactylogyrus pterocleidus Gussew from the gills of Erythroculter 

 oxycephalus Bl. from the Island of Hanka; B--Urocleidus acer Mueller 

 from the gills of Eupomotis gibbosus L. from the U.S.A. (According to 

 Mueller, 1936); C--Pricea sp. from the gills of Cybium guttatum (Gil. ) 

 from the Java Sea. 



hooks, whereas in the second--2. However, in the last case these two 

 plates are connected constantly w^ith each other by means of special out- 

 growths and apparently without any capability of movement^ forming a 



I 



We did not have live material at our disposal, only fixed. 



single two-faced plate which in the end (in the last analysis, nobis) plays the same 



role as the connecting plate of Tetraonchus with which it has a considerable exterior similarity. 



One must attribute also the indubitable similarity in structure of the disc of 

 Acanthocotylidae and of the "plectans" (or plaque, nobis) of Diplectanidae to 

 the same category of facts (see page384 ). At the same time, the similarity is 



559 



