CHITONID:. 245 
more subrotund. The auricles receiving blood from the bran- 
chial veins communicate in the usual lateral manner with the 
greater division of the heart, and at this point present their 
largest calibre; they then form an attenuated arcuation on 
each side the constricted portion, and effect a second contact 
at the sides of the lesser inflation, which may be regarded as 
an aortic ventricle receiving the blood by a special auricular 
apparatus. From the anterior axis of the major part of the 
heart, a long and large aorta or arterial vein ascends medially 
to the front, distributing by diverging arteries the aérated 
fluid to the greater portion of the body, whilst the mmor and 
posterior ventricle, after receiving from the arcuated ducts of 
the auricles its quota of blood, serves the remaining area; but 
I could not detect a descending arterial vein, which however 
must exist to convey the blood; unless one of the arteries of 
the ascending aorta doubles back and supplies the part. But 
this conjecture is scarcely tenable, as we must then suppose, 
that the blood conveyed by the arcuation of the auricles to the 
lower inflation is thrown back again on them and the larger 
ventricle, thus producing a useless periodic action and counter- 
action. 
We will endeavour to explain the probable causes and effects 
of this unusual construction. It has been shown that the 
principal ventricle of the heart is the propelling power of the 
blood to the major part of the body, and the smaller one by 
its separate auricular contact supplies the remainder ; we have 
also observed on the connection of both parts of the heart by 
the strangulated and without doubt valvular intervening por- 
tion, so that no blood can pass between the two; and it is 
clear that none is intended to pass, as the auricles by distinct 
ducts supply each with its proportion of the vital fluid; con- 
sequently the mitral valves of the ventricles are closed at their 
axes abutting on the strangulation at each simultaneous dila- 
tation, and thus this constriction acts as a point d’appui and 
of departure, and enables them to exercise a full power of 
propulsion. 
We shall now perceive the reason for these separate infla- 
tions. If one only had existed, with an ascending and de- 
