^ Sir Everard Home's account of the 
nicatiom between the artery and vein, which being kept 
full, furnish a supply of blood to be used when necessary s 
and admit of a greater or less proportion going to the head, 
or being returned by them to the vein, as occasion may 
require, their coats being exceedingly elastic. 
From these observations on the circulation in the Lumbricus 
marinus, and Lumbricus terrestris, and those formerly made 
on that of the Teredo navalis, these genera appear to form 
three links in the chain of gradation of animals, and have led 
me to the belief, that the striking difference between the cir- 
culation of the blood in all the vermes, and that of the 
higher classes of animals, may be explained, and shown to 
answer an essential purpose in their oeconomy. 
In explaining my opinion, I shall make myself better under- 
stood, by reviewing in a summary manner the modes by which 
the circulation is carried on in the different classes of animals; 
this will also enable me to show that a classification of ani- 
mals will at least be as perfect by taking the circulation of the 
blood for our guide, as the brain and spinal marrow. 
In all animals of the class Mammalia, there is a complete 
double circulation; in the one, the blood is aerated; in the 
other, the body is supplied ; they correspond in velocity, the 
aeration is great, the heat of the animal is kept up to a 
certain degree, and, if the action of the heart both in the 
auricles and ventricles has once entirely ceased, it cannot 
be restored. In birds, the circulation is completely dou- 
ble, but the aeration of the blood is less than in the mam- 
malia, the lungs being smaller, and their cells larger ; when 
the action of the heart has entirely ceased, it cannot be 
restored. 
