220 SNAILS AND SLUGS [CH. VII 



when this muscle contracts the rectum is thereby shortened 

 and its contents driven out, usually with a decided spiral 

 movement. 



Circulation and Respiration. The blood of Helix 

 performs both respiratory and nutritive functions 1 ; accord- 

 ingly we find the circulatory system well developed. The 

 heart beats at the rate of about 58 pulsations per minute 

 in an adult Helix hortensis, but more rapidly in young 

 individuals. The rate is however variable in accordance 

 with temperature, muscular exertion, and similar in- 

 fluences. The auricle of the heart receives aerated blood 

 from the pulmonary vein and drives it on into the 

 muscular pear-shaped ventricle. The passage from auricle 

 to ventricle is guarded by a valve which prevents the 

 blood from flowing back to the auricle. From the 

 ventricle the blood passes into the main aorta, which after 

 a short course bifurcates into the anterior and posterior 

 aortse : the former of these distributes blood to the salivary 

 glands, anterior part of the foot, buccal mass and head ; 

 the latter to the other digestive and to the reproductive 

 organs. The branches of these main arteries are com- 

 paratively few and debouch into irregular spaces among 

 the tissues. These spaces eventually unite either with 

 the large visceral sinus which runs from the apex of the 

 visceral coil, or with one of the two sinuses lying at the 

 sides of the pedal gland. From these large sinuses the 

 blood, now charged with carbon dioxide gas and to a great 



1 Milne Edwards, Mem. Acad. Sci. Paris, xx. ; Girod, 2nd Internat. 

 Congr. Zool. Moscow, Part 2, 1892; Natepa, Sitzb. Acad. Wiss. Wien, 



LXXVII. 



