70 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



The renal organ is placed posterior to the heart, between the passage 

 leading from the pulmonary cavity to its orifice, and the course of the 

 rectum. 



THE GENERA OF DISINTEGRATED HELIX. In this genus, as is also the 

 case in all the testaceous genera under examination, the pulmonary 

 chamber is comparatively very large, for reasons already stated, and 

 occupies a position on the outside of the lower one or two whorls of the 

 turbinated mass of the viscera. In front, it is bounded by the collar, 

 in the right side of which is the pulmonary orifice. The floor of the 

 cavity is formed, as in slugs generally, by the muscular peritoneum. 

 The roof, or outer wall, is occupied on the right side by the rectum, 

 posteriorly by the heart and renal organ, and anteriorly by the pulmo- 

 nary 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 pul- 

 monary 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 peculiar 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 granulated, 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 tessellated epithe- 

 lium ; and the exterior of the former and interior surface of the pericar- 

 dium 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 peculiar fluid very freely, the 

 pericardium frequently appearing distended with the liquor pericardii. 

 In the testaceous genera it frequently contains numbers of an entozoon, 

 which Dr. Leidy has named Distoma vagans. 1 The blood-vessels, espe- 

 cially in the liver, exhibit a white opaque appearance, which is depen- 

 dent upon the deposit in the sides of the vessel of innumerable, oil-like 



granules. 



i See Journal Acad. Nat. Sci. PLila., New Series, Vol. I. 



