162 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 117 



may have arisen. Careful comparison of antennae, mouthparts, 

 gnathopods, lu-opods, and telson indicates few basic differences between 

 Pontoporeia and Gammarus, a similarity that is reinforced by the pro- 

 nounced sexual dimorphism and similar mating behavior in the two 

 genera. Dennell (1932) concluded that, aside from special secondary 

 modifications of maxUlae and maxillipeds, the mouthparts of Hau- 

 storius arenarius strongly resembled those of Gammarus locusta. Only 

 the relatively elongate peraeopod 4, the strongly expanded basos of 

 peraeopod 5, and acute anterolateral headlobes of the Haustoriidae 

 consistently separate members of this family from typical Gam- 

 maridae. Ecologically, the two families overlap considerably, with 

 both represented mainly in shallows of marine, brackish, and fresh 

 waters, and with relatively few species in the deep benthos. 



The Gammaridae may now be considered to encompass the proto- 

 type or the morphologically most primitive species of existing Amphi- 

 poda. In the fresh-water Gammarus lacustris, for example, the mouth- 

 parts and limbs are completely and regularly segmented, fully de- 

 veloped and fully setose, and are the least differentiated or specialized 

 of all known gammarids. The Gammaridae is essentially a fresh- 

 water family, with two-thirds of the genera listed in Barnard (1958) 

 being nonmarine. The family is mainly cold-temperate in the north- 

 ern hemisphere. By contrast, aquatic members of the highly evolved 

 and land-adapted Talitroidea largely occupy the relatively warm fresh 

 waters of the southern hemisphere. 



The morphologically most specialized and highly evolved Gam- 

 maridae (e.g., Melita, Ceradocus) are fully marine, warm-temperate or 

 deep-water pelagic forms. Similarly the Haustoriidae are essentially 

 shallow-water, cold-temperate animals, and are represented in fresh- 

 water and coldest (high latitude) marine regions by the primitive 

 genus Pontoporeia, at intermediate temperatures and latitudes by the 

 more advanced Amphiporeia, Bathyporeia, and Priscillina, and along 

 the warm, low-latitude marine shores by the highly evolved 

 Haustoriinae. 



The construction of a hypothetical family tree within the Hau- 

 storiidae presents some difficulty because of the likelihood of parallel 

 evolution and morphological convergence in groups of different origin. 

 This phenomenon is illustrated on a broader basis by the fossorial 

 form of Priscillina, a form that has evolved independently among 

 such diverse gammaridean families as the Gammaridae (e.g., Ponto- 

 gammarus), the Lysianassidae (e.g., Tmetonyx nohilis), the Phoxo- 

 cephalidae (e.g., Paraphoxus), and the superfamily Talitroidea (e.g., 

 Dogielinotus) . Reliable indices of their true familial relationships are 

 provided by the form of certain mouthparts, especially the mandible 

 and lower lip, by the shape and armature of the female brood plates. 



