of the separate chitinous parts of the system of the secondary discs 

 among different species of Diplectanum we see that they represent 

 separate hook-shaped thorns with more or less well-developed parts 

 and that in a number of cases they acquire rather considerable sizes 

 and the same form as the spear-shaped thorns of Rhamnocercus 

 (Figs. 181 and 272). Examining the question about the primary or 

 secondary nature of the armature of the "discs" and the spear-shaped 

 hooks, we are inclined toward the opinion that the latter are derivations 

 of the former. As a basis for this serves indirect evidence which, 

 however, in our opinion, is sufficiently convincing. Thus, the nature of the 

 location of the spear-shaped hooks is no less strictly determined than that 

 of the separate elements of the secondary discs. Furthermore, among 



the representatives of 

 Lepidotrema the "discs" 

 are equipped also with 

 thorns of large dimensions 

 and they are apparently very 

 similar to the spear-shaped 

 hooks. Finally, among the 

 representatives of Rhamno- 

 cercus , groups of large 

 thorns also appear in the 

 middle part of the disc and, 

 what is naost important, 

 along the sides at the places 

 Fig. 272. Rhamnocercus sp. , posterior of the location of the middle 



end of the body of the adult animal from hooks. This undoubtedly is 



the gills of Umbrina nebulosa R. from a secondary phenomenon, 



the region of New York, U.S.A. (Atlantic Thus, there is all the necessary 

 Ocean). basis to suppose that Rhamno- 



cercus became separated at 

 a later time than Diplectanum and the genera close to it. 



aoiMM 



The combination of Diplectaninae and Rhamnocercinae into 

 a separate family does not arouse the slightest doubt. As Monaco, 

 Wood and Mizelle (Monaco, Wood, and Mizelle, 1954) correctly write, 

 the uniting characters are: 1) the presence of cuticular thorns ("little 

 scales")on the body; 2) dorsal and ventral middle hooks; 3) the 



similarity of the edge hooks with the ones among Diplectanum, and p. 357 



4) the presence of connecting plates. Speaking more precisely, there 

 is complete homology between the initial attaching apparatus of Diplectanidae 

 and the one described by Monaco, Wood, and Mizelle, for the species 

 Rhamnocercus rhamnocercus, and of two species which havenotyet 

 been published by us. The problem of isolating Rhamnocercus into a 

 separate subfamily is complex. Monaco, Wood, and Mizelle write, tliat 



428 



