SPECIAL ANATOMY. 69 



organ, and heart. The pulmonary orifice is situated at the anteroinfe- 

 rior edge of the mantle, on the right side of the body. When open it 

 appears round ; it is closed by means of circular muscular fibres. The 

 pulmonary rete is principally displayed upon the roof of the cavity, and 

 from it converge three principal trunks, to form the pulmonary vein 

 which passes to the auricle of the heart. 



The heart, enclosed within a pericardium, is situated near the 

 middle of the pulmonary cavity. The auricle and ventricle are pyri- 

 form, and placed base to base. The sides of the latter are consider- 

 ably thicker than those of the former, and present internally several 

 well-marked fasciculi, crossing in different directions. Between the auri- 

 cle and ventricle is a double valve. From the apex of the ventricle 

 passes off the aorta, which pierces the muscular peritoneum, and divides 

 into two principal branches, — one passing to the sub-cesophageal gan- 

 glia devoted to the viscera in the anterior part of the visceral cavity, the 

 other passing to supply the viscera posteriorly. Upon the right of the 

 heart, attached to the roof of the pulmonary cavity, is placed a large 

 glandular organ, considered as the kidney ; from the whole of its right 

 margin pi-oceeds a duct backwards, which then curves to the side of the 

 rectum, at the left side of whichjt remains attached to the pulmonary 

 orifice. 



Arion. The pulmonary cavity is situated as in Liniax. Its whole 

 interior surface presents an intricate rete, from which converge six or 

 Beven pulmonary veins to the auricle of the heart. 



The renal organ forms a complete circle around the heart, and is per- 

 forated by the rectum, in the course of the latter to the pulmonary 

 aperture. 



Tebennophorus. The pulmonary cavity is situated beneath the an- 

 terior portion of the mantle. It is formed by an inflection of the mus- 

 cular layer of the integument of the body. The renal organ is placed 

 to the right of the heart, and at its posterior part is perforated by the 

 aorta. 



Veronicella. The pulmonary cavity is situated between the mus- 

 cular peritoneum and the integument of the body. Its principal por- 

 tion is placed upon the right side, anterior to the middle, but extends 

 to the left side, over the back, and along the right side to the pulmo- 

 nary aperture, between the tail and posterior extremity of the podal 

 disk. The heart is placed in the anterior portion of the cavity. The 

 auricle receives a vein from the right and another from the left side. 



