138 



CRUSTACEA PERACARIDA 



tioii, one species, P. femorata, being entirely marine, in the 

 Arctic and North Atlantic, P. affinis inhabiting the Atlantic, 

 and also freshwater lakes in Europe and North America, 

 P. microphthdlma being confined to the Caspian Sea, and P. loyi 

 to Lakes Superior and Michigan. 



Fam. Gammaridae. — Includes fifty-two genera. The first 

 antennae are slender, with the accessory flagellum very variable. 

 The mandibles have a dentate cutting edge, spine-row, and molar 

 surface, and a three-jointed palp. The first two thoracic limbs 

 are subchelate. This family includes a few marine, but mostly 



Fig. 9.5. — Gammarns loctista, S (above) and 9 (below), x 4. Aid.l, First abdominal 

 segment; T, telson ; TJi, seventh free thoracic segment ( = 8tli thoracic segment) ; 

 C, third uropod. (After Delia Valle. ) 



brackish and freshwater species. Crangonyx is entirely subter- 

 ranean in habitat, as is Ni^phargus, N. forelii occurring, however, 

 in the deep waters of Lake Geneva. Both these genera are blind. 

 Gammarus has thirty species, G. locusta being the common species 

 on the North Atlantic coasts, and G. pulex the common freshwater 

 species of streams and lakes in Europe. A number of Gammaridae 

 inhabit the Caspian Sea, e.g. Boeckia, Gmelina, Niphargoides, etc., 

 while the enormous Gammarid fauna of Lake Baikal, constitut- 

 ing numerous genera, showing a great variety of structure, some 

 of them being blind, belong to this family, e.g. Macrohectopus 

 (Constantia), Acanthogammarus, ITeter'ogammarus, etc. 



