208 ISOPODA NORMALIA. 
ISOPODA NORMALIA. 
THis group comprises the great majority of the order 
which are distinguished by the nearly uniform size of 
the seyen segments which compose the pereion, as well as 
of the seven pairs of legs attached to such segments. 
The cephalon is always distinct; the branchial organs are 
attached to the underside of the segments of the pleon, 
being closely connected with the pleopoda, and lie 
upon each other, being sometimes covered by a larger 
outer pair, in the more normal forms, 
The Isopoda normala constitute two principal divi- 
sions, founded upon the situation inhabited by the 
animals of which they are composed ; the majority (Aqua- 
spirantia) residing, either free or in a parasitic condition, 
upon animals which live in the water, and consequently 
breathing oxygen from the water in which they live; 
whilst those of the other division (Aero-spirantia) reside 
on the land, generally indeed in deep situations, and 
breathe the free air by means of series of branchial 
plates, probably assisted by spiracles with which the 
branchiz are furnished. 
