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 brush 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. 
Earn.— GAMMARIDiE. 
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- 
