NATURAL HISTORY. 91 
in magnitude towards their extremities; the 
next two pair are small and tapering, and the 
last six are the longest. They are all set with 
clusters of fine hair. The three pair of bran- 
chial organs which succeed the legs are in con- 
stant vibratory motion, and play between the 
lamelliform plates before described; they are 
furnished with fine bristles at their extremities, 
and along their sides: the latter are not shown 
in the drawing, nor are they easily detected in 
living specimens; I first observed them in the 
exuvie. It is worthy of observation, that the 
function of respiration in these creatures is 
performed externally. The branchial organs 
playing between the plates bring fresh portions 
of water, containing air, to be absorbed by their 
internal surfaces, answering the office of lungs 
in the vertebral animals. 
The tail consists of two caudal processes, 
terminated by spines; they are attached to the 
body by four intermediate segments, the first 
and third of which are furnished on the 
inferior side with a double appendage, as shewn 
in the drawing. 
These crustacea are very voracious, yet they 
can live for a considerable time without food. | 
put a few aquatic Moluscee and a Nais with it in 
