PTEROPODA AND GASTROPODA. 555 



cartilages, which can recede from or approximate each other, or be 

 moved together to and fro, by special muscles : by these movements 

 the spines can be made to scrape with force, the action being 

 like that of a convex saw against any opposed surface ; and it is by 

 the repetition of such movements, aided, perhaps, as Cuvier conjec- 

 tures, by a solvent property of the saliva, that the whelk effects 

 the perforations in the hard shells of other mollusca, upon the soft 

 parts of which it preys. In many other Pectinibranchs the mouth 

 has the form of a retractile proboscis {Cyprcea, Murex, Voluta, 

 Paludina, Jig. 204. p). In Doris, and most other Nudibranchs, the 

 tongue is beset with the siliceous spines. Excellent descriptions 

 and figures of the lingual dentition in the present class will be 

 found in CCCLVin. and CCCLIX. 



The salivary glands present different forms and degrees of develop- 

 ment in different Gastropods, bearing the ordinary relations to the 

 construction of the mouth and the nature of the food. They are 

 usually two in number : their ducts, which are lined by a ciliated 

 epithelium, open into the oral cavity on each side of the tongue. In 

 the CalyptrcBa I found the salivary glands represented by two simple 

 elongated secreting tubes: they also present the tubular form in 

 Aplysia, Thetys, and many Nudibranchs. In the whelk they present 

 a conglomerate structure, are situated on each side of the oesophagus, 

 at the base of the proboscis, along which they transmit their slender 

 ducts to terminate on each side the anterior spines of the tongue. In 

 the Paludina vivipara {fig. 204.) the salivary glands are shown at 

 r: their structure consists of ramose caeca. In the vegetable- 

 feeding slugs and snails, the salivary glands are largely developed 

 {fig- 207 a.) : they expand upon the sides of the stomach, partially 

 blend with each other and encompass that cavity, sending their long 

 ducts, a, forwards to the mouth, and are readily distinguishable by 

 their whitish colour. In many proboscidian pectinibranchs the sali- 

 vary glands are placed in the abdomen, and have long and tortuous 

 ducts, adapted to follow the movements of the dentated proboscis, 

 near the anterior end of which they terminate. In a few Gastropods, 

 e. g., Janthina, Actceon, Atalanta, there are two pairs of salivary 

 glands. 



A common type of the divisions and disposition of the digestive 

 canal in the Gastropods is exhibited in the common river-snail (^Palu- 

 dina vivipara, fi^j. 204.) The oesophagus q is long and slightly convo- 

 luted ; q' is the last bend which the tube makes before expanding into 

 the stomach, r : s, s' show the folds of the intestine in the substance 

 of the liver and ovary ; it penetrates the branchial chamber at s", in 

 which the rectum, t, is seen passing along the base of the pectinated gills, 



