AMTOMY OF THE SMIL 



HELIX THYROIDES. 

 By Geokge B. Simpson. 



I commenced the dissection of Helix thyroides with the intention 

 of writing a monograph similar to the one bj myself on the Ano- 

 donta jluviatilis, but other work was so pressing that I have been 

 unable to continue my investigations. I have partially illustrated 

 the pulmonary cavity. Next season I shall continue the work accord- 

 ing to the original design. 



The lungs consist of a simple cavity communicating with the 

 exterior by an orifice situated on the right side of the body, closed 

 by a sphincter muscle. The cavity is comparatively large and is 

 situated on the outside of the lower two whorls. The position and 

 extent of the cavity is shown on plate 3, figures 2, 3, 4. 



In front is the collar, in the right side of which is the pulmonary 

 orifice. The bottom or floor consists of the muscular peritoneum. 

 On the right side is the rectum, and posteriorly the renal organ and 

 heart, which is composed of an auricle and ventricle. The interior 

 of the cavity is lined with a net-work of veins, which frequently 

 anastomose and receive numerous dendritic veins. The veins are 

 most abundant near the pulmonary orifice; they connect with a large 

 vein which, commencing near the orifice, continues backward on a 

 line with the rectum and opens into the auricle. The blood is con- 

 veyed to the lungs by aorta proceeding from the ventricle. 



When the animal is living the blood vessels can frequently be 

 traced through the shell, presenting a beautiful golden appearance; 

 they might without a knowledge of their character be mistaken for 

 markings of the shell. 



The pulsations of the heart can be very distinctly seen ; they 

 usually number about fifty in one minute, though sometimes vary- 

 ing from as high as seventy to as low as thirty. 



