VASCULAR SYSTEM OF SNAIL 



441 



imperfectly divided into three. Besides producing juices 

 which digest ail kinds of food, the gland makes glycogen, 

 stores phosphate of lime, and contains a greenish pigment. 

 It is thus more than a " liver," more even than a " hepato- 

 pancreas," it is a complex digestive gland, producing several 

 digestive ferments. The phosphate of lime may possibly 

 be used to form the autumnal epiphragm. 



Vascular system. — The blood contains some colourless 

 amoeboid cells, and a respiratory pigment called haemo- 

 cyanin, which gives the oxidised blood a blue tint, and is 

 very common among Molluscs. 



The heart, with a ventricle and a single auricle, lies in a 

 pericardial chamber on the dorsal surface, to the left side, 

 behind the mantle cavity. The average number of pulsa- 

 tions in Gasteropods is about one hundred per minute, but 

 in the hibernating snail the beating is scarcely perceptible. 



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. From these 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 

 {Ltttorma), breathe by gills covered by 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 rapidly emptied when the body is withdrawn into the shell. 



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 



