VASCULAR AND RESPIRATORY SYSTEMS. 353 



Vascular System. 



The blood of the snail contains some colourless amoeboid 

 cells, and a respiratory pigment called haempcyanin, which 

 gives the oxidised blood a blue tint, and is very common 

 among Molluscs. 



The heart, consisting of a ventricle and an auricle, lies 

 within a pericardial chamber on the dorsal surface, to the 

 left side, behind the mantle cavity. 



From the ventricle pure blood flows by cephalic and 

 visceral arteries to the head, foot, and body, passes into 

 fine ramifications of these arteries, and thence into spaces 

 among the tissues. Authorities differ as to the existence of 

 capillaries, but the distinction between these and narrow 

 channels is of no physiological importance. From spaces 

 among the tissues the blood is collected in larger venous 

 spaces, and eventually in a pulmonary sinus around the 

 mantle cavity, on the roof of which there is a network of 

 vessels. There the blood is purified. Most of it returns 

 directly to the auricle by a large pulmonary vein, but some 

 passes first through the kidney. 



Respiratory System. 



Most Gasteropods, e.g., the dog whelk (Purpura\ the 

 buckie (Buccinum), the periwinkle (Littorina), breathe by 

 gills covered over by a fold of the mantle. The snail being 

 entirely terrestrial has a pulmonary or lung cavity, formed by 

 the mantle fold. On the roof of this cavity the blood 

 vessels are spread out. Air passes into and out of the 

 pulmonary chamber by the respiratory aperture. When the 

 animal is retracted within its shell, the freshening of the air 

 in the pulmonary chamber takes place by slow diffusion, but 

 when the snail extends itself at full length, the chamber is 

 rapidly filled with air, and it is even more rapidly emptied 

 when the animal withdraws into the shell again. 



Excretory System. 



There is a single triangular greyish kidney behind the 

 pulmonary chamber, between the heart and the rectum. It 

 is a sac with plaited walls, and excretes nitrogenous waste 

 products, which pass out by a long ureter running along the 



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