352 MOLLUSC A. 



leaves may be compared very roughly to that of a file, but 

 its movements within the mouth also produce a kind of 

 suction which draws food particles inwards. In this 

 suction the muscular lips and the cilia in the mouth 

 cavity assist. 



Altogether apart from the radula, lying on the upper 

 surface of the buccal chamber, sometimes visible when 

 the snail opens its mouth, is a hard, crescent shaped jaw 

 plate. 



The ducts of two large salivary glands open on the 

 dorsal surface of the buccal cavity, and there are numerous 

 distinct glandular cells close to the entrance of the two 

 ducts. The salivary glands are large lobed structures, 

 and extend far backward on the crop. They consist of 

 hundreds of glandular cells or unicellular glands, which 

 secrete a clear 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 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," 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 enterochlorophyll. It is possible 

 that the phosphate of lime may be used to form the 

 autlimnal 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, pro- 

 ducing several digestive ferments. 



