17 



increased the obligate nature of parasitism. Finally, the neoteny of several 

 hyperiidean families is beyond doubt. Generally, neoteny is widespread 

 among many groups of deepwater animals and is, obviously, one of the 

 important paths of evolution in waters at great depths, where food is 

 scarce (Walters, 1961). Naturally, neotenic animals considerably differ 

 morphologically from their normal ancestral forms. 



The different degree and different nature of adaptation to para- 

 sitism and neoteny can explain those significant morphological differ- 

 ences found in various hyperiidean groups. 



Nevertheless, we do not wish to emphasize the monophyletic 

 origin of all hyperiideans. For example, the differences between the 

 infraorders Physosomata and Physocephalata are so great that their 

 origin can be explained either by prolonged processes of evolution 

 and neoteny of Physosomata, or by what is equally probable, a 

 different origin. But we simply cannot agree with Stephensen and Pirlot 

 (1931) regarding the polyphyly of Physosomata themselves. Although 

 in structural type of mouthparts this infraorder can be subdivided into 

 two groups — ^Lanceoloidea and Scinoidea, these are distinctly mutually 

 linked and, as noted earlier, the family Archaeoscinidae combines in it 

 the characters of both groups. Still less convincing is Pirlot' s suggestion 

 about the disjunct polyphyly of several other groups of hyperiideans. For 

 example, he considers that the families Cystisomatidae, Vibiliidae, and 

 Paraphronimidae, which are fairly close to the infraorder Physosomata, 

 nonetheless have another root in the suborder Gammaridea, because 

 their mandibles retain the dental process, which has disappeared in the 

 Physosomata. This difference can be more simply explained by the greater 

 obligate nature of parasitism of the Physosomata than of the above- 

 mentioned families. We know that even in gammarideans of one and 

 the same family, free-living genera have an excellently developed dental 

 process on the mandible, while genera that changed over to parasitism or 

 commensalism (for example, Chevreuxiella, Crybelocephalus, and others 

 in the family Lysianassidae) have totally lost it. 



Thus Pirlot' s suggestions should be considered more than hypo- 

 thetical. He hoped that later investigations of hyperiideans would allow 

 a more precise justification for the line of development proposed by 

 him. However, this has not happened. Present-day investigators are not 

 inclined to consider a finer division of the group, although they do not 

 insist on its monophyly. 



3 VERTICAL AND GEOGRAPHIC DISTRIBUTION OF 

 HYPERIIDEANS 



Although without exception hyperiideans are pelagic animals, as men- 

 tioned above, individual groups of this suborder differ in their ecology. 



