50 AMPHIPODA. 



Tribe — Natatoria. 



In this tribe the superior antennas are always longer 

 than the peduncle of the inferior pair. The third pair 

 of caudal appendages reach as far as the extremity of 

 the second pair. The hairs upon every part of the 

 animal are generally slight and flexible, never short, 

 stiff, and double-headed, as found upon the animals in 

 the tribe Saltatoria. 



The habits of the animals in this tribe are aquatic, 

 the most littoral living at half-tide under weeds and 

 stones, but by far the greatest number of the species are 

 found in the water. 



The common mode of progression is by swimming. 

 This act is performed by the constant play of the three 

 pairs of limbs succeeding the last pair of feet, which 

 thus receive the common synonym of natatory legs. 

 These are long, multiarticulate, pliable, and feathery ; 

 they brusli the water with a constancy equal to that 

 of the fins of a fish in motion, and propel the creature 

 with considerable velocity. It is from this circumstance 

 that the name of the tribe has been derived. If acci- 

 dentally thrown upon dry land, they have neither the 

 power to walk nor to leap — they consequently wriggle 

 along upon one side, a circumstance which has obtained 

 for them the familiar cognomen of " sea-screws." This 

 tribe contains but a single family. 



Fam.— GAMMARID^. 



The antennae are well developed, and generally sub- 

 equal. The inferior pair are inserted in a notch at the 

 infero-anterior angle of the cephalon, with which, how- 



