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 Colwmellw or 
central column, as the spiral staircases in the 
Crystal Palace. 
