22 SYNOPTIC KEY 



ii(io) Adults with four pairs of legs (certain young forms 

 with three or two pairs of legs, in which case head, 

 thorax, and abdomen are all fused), mostly terrestrial, 

 some aquatic; respiration usually by means of tracheae 

 or lung books or both or in the smaller aquatic forms, 

 through the skin; by gills only in Limulus. 12 



12(13) With four pairs of legs. 



Class Arachnida adults, p. 25 



13(12) With three or two pairs of legs. Class Arachnida, 

 young and one adult gall-producing form. ' p. 25 



14(1) With two pairs of appendages in front of the mouth. 

 Respiration by means of gills or in some small forms 

 through the skin. Mostly aquatic. In some sessile 

 and parasitic forms, the appendages may become 

 modified or disappear in the adult. 



Class Crustacea, p. 22 



Class CRUSTACEA 



1(2, 3) Usually of a small size. Body very commonly divided 

 into an anterior region bearing appendages and a 

 posterior region without appendages. When append- 

 ages are present, three are always more than three 

 pairs. Body often more or less completely covered 

 with a carapace. (In the adult stages of some attached 

 parasitic forms the anterior appendages are modified, 

 reduced or absent, or all appendages are absent.) 



Subclass Entomostraca, p. 23 



2(1, 3) Usually of considerable size. Head region consisting 



of five segments fused together but bearing five 



pairs of appendages, three of which are mouth parts. 



Thorax of eight segments some of which are commonly 



fused with the head; all bear appendages. Abdomen 



of seven segments all more or less fused (eight in 



certain marine forms). All except the last bear 



appendages normally. 



Subclass Malacostraca, p. 24 



