122 



ZOOLOGY 



APPENDIX TO CHAPTER YII 



KEY TO THE SIX CHIEF ORDERS OF MALACOSTRACA 



Body segments, 20 ; abdomen with 7 segments ; 



at least 1 pair of maxillipeds. 

 61. Cephalothorax with carapace, at least 2 pairs 



of maxillipeds [Thoracostraca]. 

 Ci. Eyes stalked. 



d\. No uncovered thoracic segments ; 3 

 or 2 pairs of maxillipeds, and 5 or 

 6 pairs of legs .... 



<7 2 . 3 free thoracic segments ; 5 pairs of 



maxillipeds ; 3 pairs of legs 

 c 2 . Eyes not stalked ; 4 to 5 thoracic seg- 

 ments not covered by carapace ; 2 pairs 

 of maxillipeds ; 6 pairs of thoracic legs 

 b'2- Cephalothorax without marked carapace ; 

 usually 7 free thoracic segments ; only 

 1 pair of maxillipeds ; eyes not stalked 

 [Arthrostraca]. 



d. Body usually broad ; abdomen with short, 

 often-fused segments, and with gills 



on legs 



c 2 . Body laterally compressed ; abdomen 

 mostly elongated, with 3 pairs of swim- 

 ming legs, and behind them 3 pairs of 

 springing legs ..... 

 o. Body segments, 21, enclosed in bilobed shell, 

 abdomen with 8 segments ; the last 2 without 

 appendages ; no maxillipeds [Leptostraca] 



Podopthalmata 



(Ex. Crayfish) 



Stomatopoda 



o 



Cnmacea 



Isopoda 



Amphipoda 



Neb ali <v 



The crayfish belongs to the order Podopthalmata. The following is a 

 key to the most important families of this order : 



i. 3 pairs of maxillipeds which differ in form from 

 the following 5 pairs of thoracic legs ; the latter 

 are locomotor, and often end in pincers [sub- 

 order Decapocla]. 



