DEFINITION OF THE TERM INSECT. 29 



tives, mostly with branching nervures containing 

 air-vessels ; found in the majority of the class. 

 Oigans [external) of respiration. A double set of 

 lateral spiracles, some for expiration. 



Abdomen. Principal seat of the organs of generation. 



Organs of motion. In the Myriapods many pairs 

 of acquired legs ; in the Thysanura elastic ventral 

 or caudal appendages. 



Organs of respiration. A double series of lateral 

 spiracles for inspiration in the majority : in some 

 only a single series, and in others only a single 

 pair. 



Organs of gerieration those common to the Ver- 

 tebrata, but retractile within the body, attended 

 usually by various anal appendages, particularly 

 a forceps in the males, and an ovipositor in the 

 females. 



Second Definition — From their internal Organization. 



Sensation. 



Nervous System. A small brain usually subbiiobed, 

 crowning a knotty double medullary chord ; nerves 

 proceeding from the brain and other ganglions to 

 all parts of the body. 

 Circulation. 



Heart replaced by a simple alternately contracting 

 dorsal vessel or pseudocordia, without arteries or 

 veins, but filled with a white cold sanies. 

 Respiration. 



Lungs replaced by tracheae, which receive the air 

 from the spiracles, and distribute it by bronchia? 

 infinitely ramified. 



