40 U.S. NATIONAL MUSEUM BULLETIN 271 



unique antenna 1, with very short and often telescoped articles 2-3. 

 Otherwise they have radiated into species that exhibit all forms of 

 inquilinous specialization such as piercing and sucking mouthparts, 

 coalesced urosomites, reduced uropod 3 and into numerous highly 

 successful pelagonts. The usual nestling and burrowing members also 

 occur in profusion. Some pelagic members are predatorial and some 

 may have developed narcotizing glands. Even wood boring is a pre- 

 sumed habit of some deep-sea lysianassids eating waterlogged coco- 

 nuts. But none has developed the tubicoly of isaeids. 



The morphologist cannot as yet recognize any clear links between 

 the Lysianassidae and other gammarideans. There are vague resem- 

 blances to Stegocephalidae, perhaps of convergence and suggesting that 

 Lysianassidae may have developed, like Stegocephalidae, first as a 

 pelagic group that later reinvaded the benthic realm. The obligatorily 

 pelagic genera among the Lysianassidae are very highly specialized but 

 many unspecialized lysianassids of other large genera (e.g., Orchomene) 

 occur prominently in the nekton. Elongation of article 3 on gnathopod 

 2 is not fully unique to the Lysianassidae, as seen in section B of the 

 written key to families (p. 109). The functional morphology of this 

 lysianassid appendage, with its other unique characters, should be a 

 problem of major concern to gammaridean evolutionists. It is not 

 grossly dimorphic in the two sexes and seems wholly inadequate, like 

 gnathopod 2 in so many other families, of serving as a sexual grasping 

 appendage. Male gnathopod 2 does serve this function in those few 

 Amphipoda that have been observed in amplexus. 



Limnetic Evolution of the Gammaridea 



That Gammaridae collectively represent the most primitive mem- 

 bers of the Gammaridea and of the living Amphipoda, has some 

 support by knowledge of the occurrence of an enormously diverse 

 faunule in Lake Baikal. Nearly 250 species of Amphipoda live in that 

 Siberian lake (see Bazikalova, 1945; Dybowsky, 1874). One might 

 presume from the extent of radiation that has occurred in the Baikalian 

 gammarids that they entered the lake shortly after its formation in 

 the Triassic. All Gammaridea now living in the lake are classified 

 with the family Gammaridae even though there is justification in re- 

 garding some of the genera, like Hyalellopsis , as members of novel 

 families. Baikalian Gammaridae strikingly reflect many of the morpho- 

 logical conditions now allotted familial importance in marine amphi- 

 pods. Non-Baikalian epigean and troglobitic amphipods also show 

 some of these conditions. The diversity of Amphipoda in Baikal is 

 an enigma in the sense that the human observer has the tendency to 

 allot a great deal of time for the evolution of such a rich display of 



