440 



PHYLUM MOLLUSCA 



Alimentary system.— In cutting a piece of leaf, the snail 

 uses two instruments — the crescentic jaw-plate on the roof 

 of the mouth, and the toothed ribbon or radula on the floor. 

 This radula is like a flexible file — a short and broad strip 

 of membrane, bearing several longitudinal rows of minute 

 chitinoid teeth. It rests on a cartilaginous pad on the floor 

 of the mouth cavity, and is moved (backwards and forwards, 

 and up and down) in a curve by protractor and retractor 

 muscles. The whole apparatus, including teeth, mem- 

 brane, and pad, is called the odontophore. The radula 

 wears away anteriorly, but is added to posteriorly within a 

 radula sac which projects from the floor of the buccal cavity. 

 Its action on 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. 



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. 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 eflFect upon the food. Similar glands are 

 found in all Gasteropods, while they are entirely absent in 

 Lamellibranchs. In some boring Gasteropods the secre- 

 tion 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 respiratorv 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 



