48 COMPARATIVE ANATOMY. 



into which a white bristle has been inserted, is situated imme- 

 diately within the peristomial margin of the shell, a little to the 

 right side of the animal, but to the left of the anus, which is 

 indicated by a black bristle. 



In the lower of the two specimens, a Roman Snail (Helix 

 pomatia), nearly the whole of the shell has been removed, and a 

 red injection thrown into the auricle and pulmonary vessels. 

 An incision has been made transversely through the roof of the 

 pulmonary sac, and its interior exposed by turning back the 

 posterior flap. A black bristle has been passed through the 

 external orifice into its cavity. Attached to the displaced 

 posterior half of the sac is the pale-coloured and triangular 

 kidney, and close to it on the left side the bilocular heart. 



The floor of the respiratory cavity is non- vascular, being 

 composed of muscular fibres arranged transversely and lon- 

 gitudinally: and functionally representing a diaphragm. On 

 the right side of this floor the rectum may be seen passing for- 

 wards to the anus, which is situated to the right of, but in close 

 proximity to, the respiratory orifice. 



80. A Roman Snail (Helix pomatia), dissected to shew 

 the general disposition of its organs. 



The shell has been removed. The liver, heart, and pul- 

 monary sac have been arranged on the left side of the prepara- 

 tion, the stomach and salivary glands in the middle, and the 

 generative system mainly on the right side. 



Posteriorly to the buccal mass, and on emerging from the 

 nerve-collar through which it passes, the oesophagus expands 

 into a long and fusiform stomach. The salivary glands overlie 

 the stomach about the middle of its length, and the two ducts 

 therefrom pass forwards through the nerve-collar and pierce the 

 buccal mass anteriorly to it. Posteriorly the stomach diminishes 

 in size, and at its point of junction with the intestine receives 

 the single large hepatic duct. The first segment of the intestine 

 ascends towards the heart, and, together with the stomach, 

 makes up a curve having its concavity towards the pedal 

 ganglia. Hence its primary flexure is said to be neural. 

 Below the heart the intestine describes certain convolutions 

 in the substance of the liver, and reappearing on the extreme 

 left of the preparation, proceeds straight to the anus, which is 



