6 U.S. NATIONAL MUSEUM BULLETIN 271 



The pereon (thorax) almost invariably bears seven pairs of legs. 

 The first two pairs are called gnathopods (or gamopods) and usually 

 are prehensile, having the seventh article (dactyl) folded back on the 

 sixth article (propodus, hand, or palmar article). Rarely in gnathopod 

 1 is there sexual dimorphism but the male often has greatly enlarged 

 second gnathopods. Gnathopod 2 is beheved to be used primarily for 

 grasping the female during copulatory amplexus. The male mounts the 

 dorsal side of the female, projects the gnathopods around her body and 

 hooks them into her fifth coxae. The pair of animals then swims, darts 

 among algae, or rests until the female molts (possibly as much as three 

 days after amplexus), at which time the male emits spermatophores 

 (sacs of spermatozoa) that pass from the ventral side of his seventh 

 pereon segment into the pouch formed by the female brood lamellae. 

 Ecdysis of the female is taking place during this time period and im- 

 mediately thereafter she lays eggs through two genital pores on the 

 sternite of thoracic segment 5. The pores normally are so small and 

 heavily chitinized that the eggs cannot be laid until ecdysis occurs and 

 the pore openings become soft and phable. How the spermatophores 

 pass from male to female is unknown but they may be assisted by 

 gnathopods or pereopods. The ventral side of pereonite 7 in the male 

 bears two minute penial projections, often spinose. Occasionally they 

 are hidden by small gills attached to the coxae of this segment. 



Within a genus of gammaridean amphipods, the taxonomic recogni- 

 tion of species often depends on the shape of male gnathopod 2; hence, 

 it is difiicult to identify females specifically because taxonomists have 

 not studied minute differences in females and made them basic to 

 identification. The second gnathopods of juveniles and females often 

 are ahke; during maturation the male second gnathopods commence an 

 an increase in size and a morphological differentiation, with changes 

 taking place during each instar, even long after the attainment of 

 sexual maturity. This has resulted occasionally in taxonomic confusion 

 because some of these instars have been described as distinct species. 

 A few species are known to have radically distinct phenotypes espe- 

 cially in the terminal male. The ubiquitous Jassajalcata has dozens of 

 forms, some of these even bridging the concepts of two or more genera. 



All thoracic appendages have seven articles (segments), the 

 proximal member of which is the coxa or sideplate. A few gammarideans 

 (e.g., Bateidae) have reduced numbers of articles on various thoracic 

 appendages. Coxae are of greater taxonomic importance in gammar- 

 idean Amphipoda than in other Malacostraca and in many species 

 are structurally more an integral part of the trunk than simply an 

 article of the appendage. They resemble ventral pleuron-hke extensions 

 of segments and so contribute to the appearance of lateral compression 

 in the body plan. They are numbered from one to seven, with numbers 



