ARTHROPOD A. 127 



1. CRUSTACEA. Animal more or less truly aquatic ; respiration by gills, 

 or by the general surface of the body ; two pairs of antennae (feelers) ; loco- 

 motive appendages more than eight in number, borne by the segments of the 

 thorax, and usually of the abdomen also. 



2. ARACHNIDA. Respiration aerial, by pulmonary sacs, by air-tubes 

 (tracheae), or by the general surface of the body ; head and thorax amalga- 

 mated ; antennae ( as such), absent ; legs eight ; abdomen without jointed 

 appendages. 



3. MYRIAPODA. Respiration by air-tubes (tracheae) ; head distinct ; re- 

 mainder of the body composed of nearly similar segments ; one pair of 

 antennae ; legs numerous. 



4. INSECTA. Respiration by air-tubes (tracheae) ; head, thorax, and 

 abdomen distinct ; one pair of antennae ; three pairs of legs borne on the 

 thorax ; abdomen destitute of limbs ; generally two pairs of wings on the 

 thorax. 



