

INTRODUCTION. 19 
vessels are filled with air, but it does not disseminate itself in them 
directly, and it is only through the walls of these tubes that the 
contained gas is exchanged for the air held in suspension by the 
surrounding water. The oxygen contained in the water passes 
through certain very permeable membranes of the gill and pene- 
trates the trachee, which discharge, in exchange, carbonic acid, 
which is the gaseous product of respiration. 
Fig. 14 represents the gills or breathing apparatus in an aquatic 
insect. We take as an example the 
Eiphemera.* Jt may be observed that 
the gills or foliaceous laminz are placed 
at the circumference of the body, and 
at its smallest parts. — 
We have now seen that the respi- 
ratory apparatus is considerably de- 
veloped in insects; itis, therefore, easy 
to foresee that those functions are most. 
actively employed by them. In fact, 
if one compares the oxygen they im- 
bibe with the heavy organic matter 
of which their body is composed, the 
amount 1s enormous. 
Before finishing this rapid examina- 
tion of the body of an insect, we shall 
have to say a few words on the 
nervous system. 
This system is chiefly composed of a 
double series of ganglions, or collections 
of nerves, which are united together by 
longitudinal cords. The number of 
these ganglions corresponds with that 
of the segments. Sometimes they are 
at equal distances, and extend inachain 
from one end of the body to the other; rig. 14.—Branchie or gills of an aquatic 
at others they are many of them close We PS aaah 
together, so as to form a single mass. 
The cephalic ganglions are two'in number; they have been 























A, foliaceous laminee or gills. 
* May-fly family.—Eb., 
C2 

