160 MOLLUSCA. 
observed in birds. The third liver is placed at the posteri- 
or end of the gizzard, into which it pours its contents by a 
short duct. 
The most remarkable feature of the circulating system, 
is the position of the lungs at the posterior extremity of the 
body, which occasions a corresponding arrangement in the 
connecting organs. The entrance to the pulmonary cavity 
is immediately above the anus. The vessels in which the 
blood is aérated, are distributed on the roof and sides of 
the cavity. The pulmonic veins consist of two receptacles, 
one on each side, extending nearly the length of the body, 
which may considered as vene cave. These receive the 
blood by numerous vessels, and convey it directly to the 
lungs. The aérated blood is conveyed by a systemic vein 
into a large auricle, seated in front of the lungs, of consider- 
able size, with the walls fortified on the interior by branch- 
ed ligaments. The ventricle is placed at its anterior ex- 
tremity, and separated by two valves. The aorta arises 
from the opposite side of the ventricle, its main trunk pass- 
ing on towards the head. 
The male and female organs of generation, although oc- 
curring in the same individual, appear to occupy different 
parts of the body. The opening of the male organs is at 
the tentacula, which leads to a cavity terminating in two 
unequal recesses. The anterior is the smallest, and receives 
the termination of a vessel three or four times longer than 
the body, which takes its rise at the external base of the 
cavity, apparently from the cellular substance, and, after a 
variety of convolutions in the neighbourhood of the mouth, 
opens into the recess. The second recess is the largest, and 
the vessel connected with it is most complicated. Its ori- 
gin is in amass which occupies a considerable portion of the 
