24 SYNOPTIC KEY 



6(i) Body of free-living animals sessile and inclosed in a 

 calcareous shell or parasitic on decapods or molluscs; 

 marine. (Pratt, 358.) Order Cirripedia 



Subclass MALACOSTRACA 



1(2) Abdomen composed of eight segments, carapace 

 present covering all the body save the four posterior 

 abdominal segments; marine. (Pratt, 364.) 



Division and order Phyllocardia 



2(1) Abdomen consisting of seven segments or less. 3 



3(6) Carapace rarely present; the first and, more rarely, 

 also the second thoracic segment fused with the head: 

 eyes sessile or sometimes with a short stalk; marine, 

 fresh water, or terrestrial. 



(Division Arthrostraca.) 4 



4(5) Body usually dorsoventrally flattened; only the first 

 thoracic segment fused with the head; carapace 

 absent. Abdominal appendages, when present, flat- 

 tened, serving as gills; marine, fresh-water, and land 

 forms; free living or parasitic. Some of the parasitic 

 forms have undergone great degeneration and their 

 systemic position can be determined only by a study 

 of development. (Pratt, 372; W. and W., 841.) 



Order Isopoda 



5(4) Usually laterally compressed animals; usually only 

 the first, sometimes also the second, thoracic segment 

 fused with the head; carapace absent; gills borne on 

 the thoracic appendages; free living, rarely parasitic. 

 (Pratt, 365; W. and W., 842.) 



Order Amphipoda 



6(3) Carapace present, covering more or less of the thorax. 



(Division Thoracostraca.) 7 



7(12) Carapace does not cover entire thorax; almost all 

 marine forms. 8 



