208 ISOPODA NORMALIA. 



ISOPODA NORMALIA. 



This group comprises the great majority of the order 

 which are distinguished by the nearly uniform size of 

 the seven 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 normalia 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-spiraiitia) reside 

 on the land, generall}'^ indeed in deep situations, and 

 breathe the free air by means of series of branchial 

 plates, probably assisted by spiracles with which the 

 branchiae are furnished. 



