VASCULAR SYSTEM. 387 
of glandular cells or unicellular glands, which secrete a clear 
fluid. 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 hzemo- 
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 
