IV.] t-HE COMMON SNAIL. 275 



storehouse of combustible fatty carbo-hydrate material and 

 as a centre for secretion of a digestive ferment. 



The pulmonary sac or mantle arises as a fold of the 

 body wall, in which pulmonary vessels appear during de- 

 velopment. At the hinder end of the enclosed pulmonary 

 chamber there are situated, side by side, the heart and 

 kidney. The heart is enclosed in a definite pericardium, 

 the floor of which is in open communication with the ex- 

 cretory organ by means of a short ciliated reno-pericardial 

 duct. The excretory organ itself lies altogether to the right 

 side of the body and debouches on to the exterior by a 

 long duct, running parallel with the rectum. 



The heart consists of a single auricle and ventricle, the 

 valves between them being so disposed as only to admit of a 

 current passing from the lung sac to the body. It therefore 

 transmits only aerated blood, and as it is in no way con- 

 cerned with the propulsion of the blood to the respiratory 

 organs it is termed — like that of the Crayfish already con- 

 sidered — a systemic heart. The ventricle gives origin to a 

 single aorta which, on entering the body-cavity, subdivides 

 into two branches. The anterior of these supplies all parts 

 of the body which lie in front of the heart, and the posterior 

 is restricted to the visceral sac and its contents. These 

 arterial trunks break up into minute ramifications, which 

 pass either into capillary systems or lacunar spaces, all of 

 which converge, directly or indirectly, towards a great sinus 

 which lies at the base of the pulmonary sac. From this, 

 afferent pulmonary vessels arise on all sides ; the branches 

 , of these, reuniting in the substance of the lung-sac, form 

 a system of efferent pulmonary vessels, which unite to 

 form a large pulmonary vein which enters the heart. The 

 efferent pulmonary vessels of the right side pass, on their 

 way to the heart, through the excretory organ, in the sub- 



18—2 



