ARTICULATA. 2838 
of a sting, as in the Bee, or of an ovipositor, or “borer,” 
as in the Ichneumon, by means of which the eggs are de- 
posited in suitable places. 
Cephalization is carried to its maximuin in this class, 
and we have animals of the highest instincts under the 
articulate type. The “brain” is formed of several gan- 
glia massed together, and lies across the upper side of the 
throat just behind the mouth. The main cord, which lies 
along the ventral side of the body, with a swelling for 
each segment, corresponds to the sympathetic system of 
Vertebrates. A true brain and spinal cord are unrepre- 
sented among invertebrated animals. The digestive ap- 
paratus consists of a pharynx, gullet (to which a crop is 
added in the Fly, Butterfly, and Bee tribes), gizzard, stom- 
ach, and intestine. There are no absorbent vessels, the 
chyme simply transuding through the walls of the canal. 
The blood, usually a colorless liquid, is driven by a chain 
of hearts along the back, z. e., by a pulsating tube divided 
into valvular sacs, ordinarily eight, which allow the cur- 
rent to flow only toward the head. As it leaves this main 
pipe (aorta), it escapes into the cavities of the body, and 
thus bathes all the organs. Although the blood does not 
circulate in a closed system of blood-vessels, as in Verte- 
brates, yet it always takes one set of channels in going 
from the heart, and another in returning. Itespiration is 
carried on by tracheee, a system of tubes opening at the 
surface by a row of apertures (spiracles), generally nine 
on each side of the body. 
The sexes are distinct, and the larvee are hatched from 
egos. As a rule, an Insect, after reaching the adult, or 
imago, state, lives from six months to a few hours, and dies 
after the process of reproduction. Growth takes place 
only during larval life, and all metamorphoses occur then. 
Among the social tribes, as Bees and Ants, the majority 
(called “ workers”) do not develop either sex. 
