i66 REPORT OF NEW JERSEY STATE MUSEUM. 



b. Three posterior thoracic segments not covered by carapace ; abdomen 



very large ; five anterior thoracic limbs formed as maxillipeds, of 



which second verj- large and forms defensive weapon, and last three 



small biramous legs. stomatopoda 



bb. Thorax covered by carapace. 



c. Thorax more or less completely covered by soft carapace ; all 



thoracic appendages biramous. schizopoda 



cc. Thorax united with head to form cephalothorax usually covered 



by carapace ; three anterior pairs of the thoracic limbs biramous 



foot-jaws, and posterior five pairs of walking-legs without exo- 



podites. DECAPODA 



Order ARTHROSTRAGA. 



First, and sometimes second thoracic segment joined with 

 head and bearing- maxillipeds. Remaining seven segments free 

 and bearing legs. Eyes usually sessile. 



This group contains many familiar types, though usually of 

 rather small size. Many are parasitic, others terrestrial, while 

 some few are fresh-water in habitat. These are among the most 

 numerous of all our Crustacea, and many others will no doubt 

 be added to our fauna with future studies. 



Key to the S'ub-orders. 



a. Body usually compressed or flattened from side to side ; gills carried on 



thoracic appendages. amphipoda 



aa. Body usually depressed or flattened from above downwards; gills carried 



on abdominal appendages. isopoda 



Sub-Order AMPHIPODA. 



The Ainphipods. 



Head well defined. No carapace. Eyes sessile and usually 

 compound. Thorax of seven segments. Abdomen typically of 

 six segments and a telson. First three abdominal appendages 

 fitted for swimming. Last three pairs of abdominal appendages 

 verv different from preceding ones, directed backward and fitted 

 for springing. Gills borne on inner side of basal joints of 

 thoracic legs. Eggs carried in a marsupial pouch under thorax 

 of female. 



