DEFINITION OF THE TERM INSECT. 29 
tives, mostly with branching nervures containing 
air-vessels ; found in the majority of the class. 
Organs (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 generation 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 subbilobed, 
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 pseudocardia, without arteries or 
veins, but filled with a white cold sanies. 
RESPIRATION. 
Lungs replaced by trachez, which receive the air 
from the spiracles, and distribute it by bronchize 
infinitely ramified. 
