22 U.S. NATIONAL MUSEUM BULLETIN 271 



rent knowledge is weak and open to extensive revision as we come to 

 understand the micromorphology, anatomy, and chemistry of the 

 various Amphipoda. 



On numerous occasions parallel adaptation, convergent evolution, 

 and independent evolution of morphofunctional conditions have 

 occurred in Amphipoda. These concern cyHndricalization of bodies 

 (Eophliantidae, Podoceridae, Colomastigidae), cylindricalization of 

 heads (Cheluridae and EophUantidae), dorso ventral flattening of 

 body {Temnophlias, Podocerus and Cyamidae), development of 

 domitubicolous glands (Ampeliscidae and Isaeidea), loss of max- 

 illipedal palps (Hyperiidea, Ochlesidae, Cyamidae, and some Lysi- 

 anassidae), and in a host of minor ways. But, for example, the 

 diversity does have a measure of constriction in that Amphipoda 

 have never evolved as fully as have the Copepoda into numerous 

 parasitic modes or, to our knowledge, have the Amphipoda developed 

 hosts of rapacious or errant predators in the benthic realm. Predators 

 do occur in the nektonic Hyperiidea and Gammaridea but none 

 of the former and few of the latter have returned to a benthic orien- 

 tation. Although few Isopoda have any degree of lateral compression, 

 whereas many Gammaridea do have dorsoventral depression, the 

 Isopoda would seem to be the more highly diversified because they 

 have cylindrical representatives (Astacilla) and fully evolved para- 

 sites (Bopyridae). In contrast, the Amphipoda are far more diverse 

 than certain other orders of Peracarida, such as the Cumacea and 

 the Tanaidacea. Fourteen families of Gammaridea alone are more 

 or less inquihnous. 



A microhabitational stress must exist between various Amphipoda 

 and members of the other crustacean orders and phyla, which re- 

 stricts a fuller display of genetic potential than now in existence. 

 The tendency of some Amphipoda to enter the crawhng realm of 

 Isopoda, indicates that were Isopoda extinct, Amphipoda could 

 fill many of those niches, even though rudimentarily or imperfectly. 



Various members of the Gammaridae have been considered as the 

 most primitive of living amphipods. They display most of the basic 

 gammaridean morphology but the strong development of the lateral 

 shield (Gurjanova, 1962), composed of coxae or pereopodal wings 

 in many gammarids, suggests that they have strongly differentiated 

 from a precursor lacking such a shield. If pereopodal tube-spinning 

 glands represent a secondary development in Amphipoda, then many 

 of the isaeid genera that might be considered as close to a shieldless 

 precursor, have probably undergone a secondary reversion by a 

 reduction of the lateral shield (e.g., Corophiidae). Other groups with 

 reduced lateral shield (Eophliantidae, Podoceridae, Colomastigidae) 

 apparently do not stand close to the primitive amphipod model be- 



