VASCULAR SYSTEM. 343 



fluid stuff. This travels up the ducts, and is forced, in part 

 at least, by muscular compression, into the buccal cavity. 

 While some say that this fluid converts starch into sugar 

 (after the usual fashion of saliva), other authorities deny 

 that it has any effect upon the food. Similar glands are 

 found in all Gasteropods, while they are entirely absent in 

 Lamellibranchs. In some boring Gasteropods the secretion 

 contains 2-4 per cent, of free sulphuric acid. 



The gullet extends backward from the buccal cavity, and 

 expands into a storing crop ; this is followed by a small 

 stomach surrounded by the digestive gland; thence the 

 intestine extends, and, after coiling in the visceral hump, 

 passes forward to end on the right side anteriorly beside the 

 respiratory aperture. The digestive tract is muscular, and 

 in part ciliated internally. 



A large part of the visceral spiral is occupied by the so- 

 called " liver." This gland has two lobes, each of which 

 opens by a duct into the stomach. The left lobe' is again 

 imperfectly divided into three. Besides producing juices 

 which digest all 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 hsemo- 

 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 



