298 LECTURE XXII. 



^heir living and decomposing states. The damage which the common 

 snail produces, by devouring the produce of the garden, is too well 

 known ; and, on the other hand, the common whelk preys upon its 

 congeners, nor do their strong shells defend them from its attacks. 



The mouth is bounded by fleshy contractile lips in most of the 

 Gasteropods, and these are developed, in the Haliotis, into a pair of 

 labial processes : it is likewise generally armed with horny plates, 

 trenchant or spiny, disposed either as jaws, or covering the tongue. 

 The upper lip in the snail is armed with a crescentic dentated horny 

 jaw, which is opposed by the bifid soft lip below. In the Tritonia, a 

 curved trenchant horny plate works vertically upon another of similar 

 form, and with these, as with a pair of curved scissors, this mollus- 

 cous animal crops the tough sea-weed which constitutes its food. 

 Certain fresh-water Gasteropods, as Limnceiis and Planorbis, combine 

 two lateral horny jaws with a superior dentated labial plate. The 

 Limpet rasps marine plants with a narrow horny plate or ribband beset 

 with numerous rows of minute recurved hooks, which armature ex- 

 tends beyond the mouth, and is longer than the entire body. The 

 whelk is provided with a more complicated instrument in the shape 

 of a proboscis, susceptible of considerable elongation, or of being 

 entirely concealed within the interior of the body. Its extremity is 

 vertically cleft, the divisions or lips having their inner surface beset 

 with recurved spines. In the interior of the muscular cylinder, there 

 is a tongue armed by a horny uncinated plate, as in the Limpets, but 

 of much less length : it is stretched upon two elongated 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 against any opposed surface ; and it is 

 by the repetition of such movements, aided, as Cuvier conjectures, 

 by a solvent property of the saliva, that the whelk effects the per- 

 forations in the hard shells of other moUusca, upon the soft parts of 

 which it preys. 



The salivary glands present different forms and degrees of develop- 

 ment in different Gasteropods, bearing the ordinary relations to the con- 

 struction of the mouth and the nature of the food. In the Cahjptrcea 

 I found the salivary glands represented by two simple elongated se- 

 creting tubes. I». the whelk they present a conglomerate structure, 

 are situated on each side 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 

 {Jig. 121.), here selected for the illustration of the Gasteropodous type 

 of the molluscous organisation, the salivary glands are shown at v. 

 The proboscis, or muscular parietes of the mouth, is seen at /> in its 



