» 148 NATURAL HISTORY OF WASPS. 
By such observers, however, from an examination of 
the different insects best adapted for the determina- 
tion of each single point, the theory of the circula- 
tion in insects has been fairly established. Thus, 
from the heart, on the dorsal surface of the abdomen, _ 
the blood passes to the head, as already described. 
The return of the blood is not effected by mere 
filtration between the tissues and organs, as was 
formerly supposed, but along channels quite clearly 
defined, though, from. the thinness of thei walls, 
well nigh invisible. One such venous canal is found 
in the track of the spinal cord, along the ventral 
surface of the abdomen. Another constitutes a kind 
of auricle, surrounding the heart, or ventricle, as we 
should now call it, and supplymg it with blood 
through numerous transverse slits. And the so-called 
cavity of the peritoneum in insects is really a collec- 
tion of venous sinuses, by which the blood can pass 
from the front to the back, to complete the circuit.* 
The general plan of the’ circulation in insects 
then, seems not so very different from that which 
obtains in the higher animals; involving only a diffe- 
rent mechanical arrangement. But in the process of 
blood-making, in the mode in which the heart is 
supplied with the newly made blood, and in which 
the blood itself is applied to its various uses, the 
insect economy presents great anomalies. 
In the higher animals the blood is made by a most 
elaborate process, in organs specially designed for 
that purpose. It is oxygenated and depurated at 
certain points only, where special organs are placed, 
which have a certain duty to perform in these 
* © Qyclopsedia of Anatomy and Physiology,’ Vol II, p. 976. 
