5 
circulation of the blood in the class Vermes. 
In the Amphibia the circulation is double, in appearance, 
but partially so in reality, the septum between the ventricles 
having apertures communicating from the one to the other ; 
this structure renders the mass of blood less aerated, and 
the circulation less dependent upon the organs of aeration. 
Under these circumstances, the blood varies in its temperature 
with the atmosphere, and life is carried on under very imper- 
fect degrees of aeration of the blood ; but, when the action of 
the heart has entirely ceased, although the individual muscles 
of which it is composed can be irritated to produce contrac- 
tions, for days, and weeks, yet the complete action of the 
whole organ cannot be restored. 
In fishes the circulation of the blood is not double, as in the 
higher classes. The heart is composed of an auricle and 
a ventricle ; the one is employed to receive the blood that has 
been used for the support of the body, and the other for pro- 
pelling it through the organs of aeration ; the aerated blood 
is collected into one artery, and passes to the different parts 
of the body for their nourishment, with no other impulse 
than what can be produced by the muscularity of the coats 
of the arterial system. It is true that the red blood does not 
go far towards the extreme parts. Even in this class, the 
circulation cannot be restored after the heart has been entirely 
at rest. Stories are told respecting fishes sold at market in 
North America in a frozen state, which, after they were carried 
home and gradually thawed, have been seen to move : to this 
I can very readily subscribe, since I have seen parts of qua- 
drupeds completely frozen so as to be made solid, then thaw 
and recover. I have seen blood converted into ice in its blood 
vessel, thaw and become fluid, yet afterwards coagulate. I 
have also seen a carp after the heart and viscera were removed. 
