Into the composition of the subclass enter three orders -- 

 Diclybothriidea Bychowsky, Chimaericolidea (Brinkmann) Bychowsky, 

 and Mazocraeidea Bychowsky. 



1. Order Diclybothriidea Bychowsky, ord. nov. 



Oligonchoinea having larvae with 10 edge hooks and 2-4 middle 

 hooks on the attaching disc and with 4 eyes which can be absent (?). The 

 attaching armature of the adult forms consists of 6 large, clamp-shaped 

 suckers each armed with one strongly developed edge hook and of a varying 

 number of unchanged edge hooks (capable of being completely absent) and 

 of growing middle hooks located on the special finger-shaped lower part 

 of the attaching disc. The latter can be equipped with a pair of feirly well- 

 developed muscular suckers. The cephalic end has 2 lateral sucker-shaped 

 pits not connected with the buccal opening, or the latter bears one more 

 or less adoral sucker. The eyes among adult animals exist in the number 

 of 4, or can be completely absent. The copulatory organ is equipped with 

 chitinous hooks or is completely deprived of armature. 



Parasites of Acipenseriformes and Elasmobranchii. Apparently 

 as an exception they are encountered on Holocephali (see page 410). 



The present order comprises very specialized forms in which 

 the attaching apparatus bears obvious traces of secondary simplification. 

 There is no doubt that the initial forms had an attaching disc equipped with 

 8 sucker-shaped clamps which was not differentiated into two distinctly 

 separate parts--the disc proper and the offshoot. Proofs of this were cited 

 by us in our common work with Gussew (Bychowsky and Gussew, 1950). 

 To speak of the presence among contemporary forms of an "offshoot" of 

 the disc is not correct, properly speaking, because it is the lower or 

 posterior part of the attaching disc of the larva which grows weakly in the 

 postembryonic period. On the other hand, its front or anterior part grows 

 very strongly just as takes place in the secondary disc of Acanthocotyle , 

 However, this is not a secondary disc but precisely the anterior part of the 

 larval disc because the clamp-shaped suckers arising on it are incepted at 

 the place of the location of the larval edge hooks (see 192 - 194 ). Hence 



it will be more correct to speak about the narrowed or widened part of the 

 disc as was done in our work cited above. 



Unfortunately, the development of one of the families --Hexa- 

 bothriidae, has not been studied and consequently a number of questions 

 about its systematic position rennain unclear (see page 406). In connection 

 with this, the small family-Diclybothriidae, which determines the name 

 of this order, is taken as a typical family. In favor of this selection is 

 also the fact that Hexabothriidae have a lower order of specialization than 

 Diclybothriidae and thus are morphologically more singular than the latter 

 (see, however, page 407 ). 



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