244 
LIFE, IN ITS INTERMEDIATE FORMS. 
contains in its substance coloured spots ; these, when 
minutely examined, are found to bo of a glandular cha- 
racter, and to owe their peculiar colours to a pigment 
secreted by themselves ; the pigment, so furnished, being 
therefore mixed up with the calcareous matter at the 
time of its deposition, coloured lines are formed upon the 
exterior of the shell wherever these glandular organs exist. 
If the deposition of the colour from the glands be kept 
up without remission during the enlargement of the shell, 
the lines upon the surface are continuous and unbroken; 
but if the pigment be furnished only at intervals, spots or 
coloured patches of regular form, and gradually increasing 
in size with the growth of the mantle, recur in a longi- 
tudinal series wherever the paint-secreting glands are met 
with.”* 
The shell increases in thickness no less than in dia- 
meter ; and this also is effected by the mantle ; the calca- 
reous matter being deposited, layer after layer, on the 
interior surface of the valve. There are do pigment-glands 
on the general surface of the mantle, and hence the interior 
of shells is. always white or destitute of colour, except that 
brilliant iridescence which we are so familiar with in many 
shells, which constitutes mother of pearl. This prismatic 
reflection depends on stria;, or impressed lines, of micro- 
scopic minuteness, on the surface, and can be produced 
artificially on several substances. Pearls themselves are 
merely quantities of this calcareous matter poured out in 
abundance at one spot, often to surround some atom of 
sand, and thus prevent its edges from wounding the sen- 
sitive mantle. 
' General Outline. 3S5. 
