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 arc covered by the 
same. The cells are granular, with distinct, round, or 
oval, granular nuclei, and a minute nucleolus. 
The pericardial epithelium separates its peculiar fluid 
very freely, the pericardium frequently appearing dis- 
tended with the liquor pericardii. In Helices, it fre- 
