8 Sir Everard Home’s account of the 
state, in which the circulation ceases, or what portion of them 
is in the habit of doing so, is no part of the present consi- 
deration ; that some of them do, is sufficiently well ascer- 
tained, and the mode of their reanimafion appears to arise 
from the air confined in the organs of aeration escaping, and 
fresh air being received, the effect of wffiich, probably, is to 
excite the arteries of those organs to action, and consequently 
to send a supply of aerated blood to the heart. When the gar- 
den snail is shut up in the winter, all external communication 
is excluded, and therefore for months the action of the heart 
and of the organs of aeration must have ceased ; when warmth 
and moisture are applied, the membranous films fall off, a 
globule of air that had been inclosed in the organs of aera- 
tion becomes rarified, it expands and forces its way out, and 
thus admits fresh air to be applied to the arteries of these 
organs. 
In the second order of vermes, the external organs of aera- 
tion must cease to act, whenever the body of the animal is 
inclosed in sand, and will have their action restored as soon 
as fresh sea water is applied to them. 
In the third order, in which the circulation consists only of 
arteries and veins, they also probably cease to act whenever 
the organs of aeration are not supplied, and renew their action 
whenever fresh supplies are received. 
In the fourth order, the action of the blood vessels is 
scarcely necessary for the functions of life, the air vessels 
carrying the air to the blood, and retaining a considerable 
supply. 
In the fifth order, the aeration of every part of the substance 
of the animal appears to be a substitute for any particular 
fluid having a regular circulation. 
