MARINE GAMMARIDEAN AMPIHPODA 27 



Stenopody is the presumed primordial condition of the arthropod 

 appendage. How far this logic can be carried into the higher Crustacea 

 as a precedent for any ordinal precursor is wholly philosophical as 

 far as Amphipoda are concerned. Our "basic gammaridean" already 

 has speciahzed anterior appendages, the gnathopods. A precursor to 

 Amphipoda would already have one pair of maxiUipeds. The highest 

 Crustacea, the Decapoda, have those two pairs of gnathopodal homo- 

 logues also attached to the head as maxilUpeds. Nonambulatory func- 

 tion of anterior appendages is thus a universally replicated feature of 

 Malacostraca and one might imagine that it was a fundamental cor- 

 ollary of amphipodan development. Amphipoda went one step further 

 than isopods or tanaids in having not just one pair, but two pairs of 

 functional gnathopods. With few presumably secondary exceptions, 

 the second pair became the dominant members. The loss of stenopody 

 signals the development of a nonwalking function as far as the anterior 

 amphipod legs are concerned. In some Amphipoda these changes occur 

 even in legs 3-4 ("pereopods 1-2"). 



The loss of stenopody was far from immutably fixed in amphipods. 

 In returning to a slender condition, the marks of a grasping function 

 have been left, \vith few exceptions. Return to stenopody, hence en- 

 feeblement, of gnathopods is a mixed bag among Amphipoda other- 

 mse recognized as either primitive or advanced. This suggests that 

 gnathopodal evolvement, especially of gnathopod 2, was a primary 

 crystalUzation of the amphipod plan, but that once upon the scene 

 the original enlargement alone was unnecessary for whatever function 

 the gnathopods were put to. Perhaps enlargement even went through 

 a detrimental cycle of overspecialization. 



Reduction in size of gnathopods must have come fairly early in 

 gammaridean evolution for several genera of the Gammaridae have 

 the gnathopodal size reduced. Some of the functional value of size 

 may have been replaced by increased setosity. All but two of the eight 

 families standing near the Gammaridae have the gnathopods enfeebled. 

 Liljeborgiidae retained the enlarged gnathopods, and among other 

 reasons, this fact supports the odd thesis that the inquilinous line of 

 Amphilochidae-Leucothoidae-Stenothoidae has some relationship to 

 the Liljeborgiidae. Those three famiUes, despite the presence of other 

 morphological degradations, have not lost the presence or potentiaHty 

 of large second gnathopods. The maintenance of enlarged gnathopods 

 in many members of the Eusiridae-Calliopiidae-Pleustidae is also 

 further confirmation of their strong relationships to the basic gam- 

 maridean. The position of the Oedicerotidae, unusually close to the 

 Gammaridae in the scheme of graph 1, reflects their enlarged gnath- 

 opods. But in the 10 families presumed to have evolved out of a 

 eusirid stock, the gnathopods have become enfeebled. Marine tali- 

 troideans maintain the enlarged gnathopod 2 but terrestrial members 



285-135 O - 69 - 3 



