160 MOLLUSC A. 



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 aerated, 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 venae cavae. These receive the 

 blood by numerous vessels, and convey it directly to the 

 lungs. The aerated 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 a mass which occupies a considerable portion of the 



