464 CHORDATA. 
Bioop-VascuLar SysTeM.—The heart and circulatory 
system do not show any great adaptation throughout the 
class. The heart is always four-chambered and the systemic 
arch is only found on the left. In various regions of the 
body there are developed fine meshworks of blood-vessels 
termed retia mirabilia. These are found at the bases of 
the limbs in many arboreal animals which have to hang 
from boughs, in which case they appear to counteract the 
Fig. 322.—A RETE MIRABILE. 
A, Ge.eral appearance. B, Cross-section of the blood-vessels. C, Anastomosis of 
smaller and larger vessels, (After MuRIE.) 
retarding effects of gravity upon the circulation (¢& Sloth). 
They also occur in whales, possibly for the storage of arterial 
blood to allow of a long sojourn under water. 
RESPIRATOkY SysTEM.—The lungs lie freely in the 
thoracic cavity, being completely surrounded by the pleura, 
and respiration is effected by the ribs and _ intercostal 
muscles, supplemented by the diaphragm, as in the rabbit. 
The diaphragm is foreshadowed in types like the crocodile, 
but it is typically a mammalian organ in its perfect condition. 
