copulatory organ in a majority of genera is a secondary phenomenon, and 

 consequently Hexabothrium is a more ancient species. Further, it is quite 

 probable that Heteronchocotyle conversely is a younger genus and the 

 alteration of its attaching disc is a secondary phenomenon. The last genus 

 deserves special attention because the structure of its disc provokes a 

 number of important considerations. First of all the drawings of the author 

 (Brooks, 1934) and Price (Price, 1942) hardly reflect the true relation- 

 ships between the parts of the attaching disc (Fig. 292, A). Actually it is 

 probably symmetrical and has a shape which is similar to the one drawn 

 in Fig. 292, B. From this drawing it is apparent that the largest hooks 

 are the ones lying in the posterior -most pair of suckers, following 

 according to size--one located in one of the middle suckers and the smallest 

 ones in one of the middle and two of the upper suckers. Sinnilar correlations 

 resemble very much the picture observed in the grow^th of the larvae of 

 Diclybothriidae during the postembryonic period and is analogous to what 

 takes place during the growth of the larvae of Microcotyle, Diplozoon and 

 so forth. The difference in the sizes of the hooks of the middle suckers 

 cannot play a principal role because the similar pictures of asynchronous 

 growth of the chitinous elements of the right and left halves of the disc are 



Fig. 291. Hexabothriidae (gen. et sp. nov. ) from the gills of Raja rosispinus 

 G. and Town, from the region of the western shore of S. Sakhalin (Sea of 

 Okhotsk). A- -The section of the body of the individual with a sex complex 

 (vaginal ducts start with a common trunk!); B--The section of the body of 

 an individual sex complex (each of the vaginal ducts begins independently.'); 

 C--Egg (filament and little foot!). 



490 



