ENTOMOSTRACANS. 



465 



SUB-SECTION III. 



THE ORDER OF ENTOMOSTRACA, OR ENTOMOSTRACANS. 



THE Order of Entomostraca comprises Crustacea which 

 are defective both in segments and feet as compared with 

 the preceding orders, and rank lower. They have nor- 

 mally six or five cephalic rings, the eight or nine posterior 

 ones belonging to the foot series, but three or more hind 

 pairs of these are usually obsolete. The abdomen is also 

 without appendages. This group embraces Carcinoids ; 

 Ostracoids, including Cirripeds ; Limuloids ; and Rotifers. 



The Carcinoids are very small or minute, and are repre- 

 sented by Cyclops, Caligus, and Argulus ; the last two gen- 

 era containing little crustaceans often found adhering to 

 the surfaces respectively of marine and fresh-water fishes. 



The Ostracoids have a bivalve carapax, and a short 

 incurved abdomen without terminal appendages. Ex- 



Fig. 368. 



Fig. 366. 



Barnacles, Anatifa. 



Horse-shoe Crab, 

 L imulus polyphemus. 



cepting the Cirripeds or Barnacles, which are from half 

 an inch to several inches long, and marine, they are 



20* DD 



