16 



In hyperiideans the coxal plates are more weakly developed than 

 in gammarideans. But even among the latter, the structure of the coxal 

 plates varies considerably in different families. Even among the pelagic 

 forms, it is difficult to compare the family Stegocephalidae, with a strong 

 lateral shield, and the family Astyridae or, particularly, the family Vit- 

 jazianidae, in which the coxal plates do not differ greatly from the prim- 

 itive hyperiidean type, which is typical, for example, of the Lanceolidae 

 or Chuneolidae. 



Finally, such characters as the broad flabellate uropods, typical of 

 hyperiideans, are undoubtedly an adaptation to a pelagic mode of life. 



Thus, most characters uniting hyperiideans ought to be considered 

 adaptive and, possibly, convergent, and not indicative of a common 

 origin. 



On the other hand, although almost all features characterizing hyper- 

 iideans individually have analogies in some groups of Gammaridea, not 

 a single group of the latter combines a majority of them, let alone all of 

 them. As yet, there is no real basis for agreeing with Pirlot that different 

 groups of Hyperiidea have less in common among themselves than with 

 any group of Gammaridea. 



If, following Pirlot, we accept the broken polyphyly of hyperiideans, 

 then each of their groups should be deemed to be derived from the most 

 specialized and, in addition, as a rule, parasitic groups of gammarideans. 

 At the same time, it is well known that narrowly specialized groups are 

 usually evolutionary blind ends and cannot act as the initial type for 

 further dicho-tomous adaptive variability. Obviously, hyperiideans orig- 

 inated from some fairly primitive or less specialized group (or groups) 

 of gammarideans, and then mostly evolved along the path of adaptive 

 radiation. Moreover, the nature of adaptations to a pelagic mode of life, 

 and particularly to ecto-parasitism, was identical in both suborders of 

 24 Amphipoda, which explains the convergent similarity of some highly 

 specialized taxa of the Gammaridea and Hyperiidea. A substantive reason 

 for taxonomic differentiation of hyperiideans could be that some of them 

 inhabited the deeper layers while others were surface dwellers, where 

 conditions require greater activity and more efficient protective measures 

 against enemies. In particular, among the surface-dwelling forms of some 

 Platysceloidea, the lateral shield arose as a secondary feature; however, 

 it formed not from the coxal plates, but through the enlargement of the 

 proximal articles of the posterior pereopods. 



It should be noted that most significant changes in the morphological 

 characters in crustaceans occur during a changeover from free-living to 

 parasitism. Hyperiideans constitute a group comprising essentially com- 

 mensals and ectoparasites of varying degree of obligate nature. Probably, 

 some taxa during evolution became adapted to free-living while others 



