236 ' INTRODUCTION. 



The heart, enclosed within a pericardium, is situated 

 near the middle of the pulmonary cavity. The auricle 

 and ventricle are pyriform, and placed base to base. 

 The sides of the latter are considerably thicker than 

 those of the former, and present internally several well- 

 marked fascicuH, crossing in different directions. Be- 

 tween the auricle and ventricle is a double valve. From 

 the apex of the ventricle passes oiF the aorta, which 

 pierces the muscular peritoneum, and divides into two 

 principal branches, — one passing to the sub-oesophageal 

 ganglia 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, glan- 

 dular organ, considered as the kidney ; from the whole of 

 its right margin proceeds a duct backwards, which then 

 curves to the side of the rectum, at the left side of which 

 it remains attached to the pulmonary orifice. 



Arion. The pulmonary cavity is situated as in 

 Limax. Its whole interior surface presents an intricate 

 rete, from which converge six or seven pulmonary veins 

 to the auricle of the heart. 



The renal organ forms a complete circle around the 

 heart, and is perforated by the rectum, in the course of 

 the latter to the pulmonary aperture. 



Tebennophorus. The pulmonary cavity is situated 

 beneath the anterior portion of the mantle. It is formed 



