324 MOLLUSCS. 



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

 that it has any effect upon the food. 



The gullet extends backward from the buccal cavity, and 

 expands into a storing crop ; this is followed by a 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 re- 

 spiratory aperture. The digestive tract is muscular, and in 

 part ciliated internally. 



As we have repeatedly mentioned, a large part of the 

 visceral spiral is occupied by the so-called " liver." This is 

 a digestive gland of many qualities, producing juices which 

 digest all kinds of food, making glycogen, storing phosphate 

 of lime, and containing a greenish pigment called entero- 

 chlorophyll. It is possible that the phosphate of lime may 

 be used to form the autumnal epiphragm, but the most 

 important fact is that the gland is more than a " liver," more 

 even than a " hepato-pancreas," it is a complex digestive 

 gland, which may be called " poly-enzymatic," i.e., productive 

 of several digestive ferments or enzymes. 



Vascular System. — The blood of the snail contains 

 some colourless amoeboid cells, and a respiratory pigment 

 called haemocyanin, which gives the oxidised blood a blue 

 tint. 



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.~y[.oi\. Gasteropods, e.g., the dog- 

 whelk {Purpura), the buckie {Bucdnum), the periwinkle 



