The Common Snail. 39 



which, corresponds with, what we may consider 

 as the back of the mollusk, and which is con- 

 siderably thickened, is termed the collar ; here 

 are situated the glands, which secrete the colouring 

 and other matter of which the shell is mainly 

 composed; although the substance called nacre, 

 or mother-of-pearl, is secreted in the thinner part 

 of the mantle ; it is however from the collar that 

 the growth or increase of the shell proceeds. It 

 is in accordance with certain variations in the 

 shape and disposition of this mantle and collar, 

 that shells assume such very different shapes. 

 Sometimes the whorls or spiral ridges, are pro- 

 jected or thrown far out, and this produces the 

 turbinated shell. Sometimes they scarcely rise 

 above each other, but rather spread towards the 

 sides, and then we have the discoid shape. Gene- 

 rally speaking, the whorls of a shell take a 

 direction from left to right, but occasionally an 

 opposite one; they are then called sinistral, or 

 left-handed shells; such are not common. If one 

 of the twisted shells be divided lengthways, it 

 will be seen that the inside of the whorls wind 

 in an ascending direction, round the Columella or 

 central column, as the spiral staircases in the 

 Crystal Palace. 



