238 INTRODUCTION. 



rete of capillary vessels. The pulmonary rete is most 

 developed in the vicinity of the pulmonary orifice ; and 

 from it in a line -with the latter, along the course of the 

 rectum, proceeds backward a single pulmonary vein to the 

 heart. The renal organ is elongated, pyramidal, and is 

 placed to the right of the heart and pulmonary vein. Its 

 duct commences upon the right border of the gland, 

 courses backward to the rectum, along the inner side of 

 which it passes to the pulmonary aperture. 



The remaining testaceous genera present nothing pecu- 

 liar in the character of the pulmonary or circulatory 

 apparatus. 



General Remarks. The heart, in warm weather, 

 beats about fifty-five times in a minute, but to some 

 extent appears to be under the control of the animal, for 

 if disturbed or irritated it pulsates much slower. 



In composition, the heart consists of distinctly granu- 

 lated, unstriped, muscular fibres, with oval nuclei, which 

 are hardly visible before the application of acetic acid to 

 them. 



The interior of the heart and aorta is lined with a tes- 

 sellated epithelium ; and the exterior of the former and 

 interior surface of the pericardium are covered by the 

 same. The cells are granular, with distinct, round, or 

 oval, granular nuclei, and a minute nucleolus. 



The pericardial epithelium separates its pecuHar fluid 

 very freely, the pericardium frequently appearing dis- 

 tended with the liquor pericardii. In Helices, it fre- 



