554 LECTURE xxn. 



and snail do, by devouring the produce of the garden, is too well 

 known : the whelk preys upon its congeners, nor do their strong 

 shells defend them from its attacks. 



The mouth is always anterior, and is bounded by fleshy contractile 

 lips. These are developed, in Ualiotis, 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 homy plate works vertically upon another of similar form, 

 and with these, as with a pair of curved scissors, this molluscous 

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

 fresh-water Gastropods, as Limnceus and Planorbis, combine two 

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

 rasps marine plants with a nai'row horny plate or ribband beset with 

 numerous (160) rows of minute recurved hooks (supported by the 

 tongue, ^^. 203. a)*, which armature extends beyond the mouth, and 

 is longer than the entire body. 203 



The apparatus is supported /^ ^^^^^^^ ^"^^^ g 



by two firm parts {b, b), from ^^2^ ^^v^^^S^^^* 



which arise the muscles (a, o.) yj -. ti^T?-^'^'^^^'''^'^^^-^^ ^^^ 



that work the rasp. A mag- y^ \.(° J^^^^' 



nified view of the arrange- if ^^^ ^ a 



ment of the lingual teeth is ^^^v Jj ^^^^2 



given at b. These teeth are ^^^Z-^^ ^^^^S 



amber-coloured, transparent, a b 



insoluble in acid: plainly Tongue of p«<rf/a. 



silicious in the limpet and most other Gastropods. It is only at 

 the fore part of the tongue (d) that . these teeth have the requisite 

 hardness : when worn down the part supporting them goes, and the 

 waste is supplied by the progressive growth, with concomitant 

 hardening, of the lingual plate (c) behind. The soft reserve portion 

 of the spiny tongue is found folded sub-spirally beneath the viscera 

 of the Limpet. 



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 ex- 

 tremity 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 an uncinated ribband, as in the 

 Limpets, but of much less length : it is stretched upon two elongated 



* CCCXXXVIII. 



