30 U.S. NATIONAL, MUSEUM BULLETIN 271 



closely associated with the Gammaridae, such as the Liljeborgiidae, 

 Eusiridae, and PhoxocephaUdae-Haustoriidae. Loss of trituration sur- 

 face and reduction in size of molar are universal in the Liljeborgiidae, 

 but the reasons are not yet apparent as the ecology of the group is 

 poorly known; various burrowers in the Phoxocephalidae, Haustori- 

 idae, and Oedicerotidae have smoothed-off molars and several of those 

 genera have enormously enlarged molars covered with a setular velvet. 

 Such molars are also seen in the Synopiidae. They must have some 

 relationship to the fossorial or semifossorial habit of processing mineral 

 grains. But in those large fossorial famiUes the normal mandible is 

 retained by numerous genera. 



The strongest changes in mandibular morphology occur in those 

 families presumed to be inquilines. In one way or another these 

 families have adapted to piercing and sucking or possibly to the 

 scraping of slime but even some inquilinous amphipods maintain the 

 biting and grinding functions. The mandible of Polycheria is used 

 chiefly for burrowing into the tests of tunicates for domiciliary purposes 

 rather than for feeding. Indeed, the biting adaptation is rarely lost 

 even in the strongest inquilines; it is maintained in many Acantho- 

 notozomatidae, although others of the family have those incisors 

 developed into stylets. The conformity of the acanthonotozomatid 

 and stegocephalid mouthpart bundle suggests a gross piercing func- 

 tion as if they normally attack some large sessile invertebrate. Pre- 

 sumably some of the "inquilines," like those of the greater Steno- 

 thoidea, are grazing predators, biting off coelenterate polyps or 

 consuming sponge and tunicate tissues. 



The pardaliscid and stilipedid mandibles are the most paradoxical. 

 They are elytriform like those of some stegocephalids, lack molars 

 but retain palps, yet the mouthpart bundle is rarely coniform and 

 no one has demonstrated an inquilinous behavior. Many of these 

 species are nekters or demersal members of the deep-sea fauna. 



Maxillipedal changes mark one of the primary subordinal grades of 

 evolution within the Amphipoda. The loss of palps is a condition of 

 the Hyperiidea. Only two families of Gammaridea and a few genera 

 of two other families have a marked reduction or loss of these palps. 

 Such loss is associated with a nektonic, often inquilinous habit, but 

 numerous pelagic Gammaridea have fully developed palps. Reduction 

 in maxillipedal plates or palps is not perfectly correlated with the 

 inquilinous families or those marked by mandibular changes,but as we 

 should expect, all stages of the perfection of this morphology are ap- 

 parent and the trend is obvious. Plates and palps often evolve in- 

 dependently as if their functions were distinct; in some cases such as 

 Liljeborgiidae and throughout the greater Stenothoidea, the plates 

 become reduced while the palps are maintained or increased in size. 



