244 



LIFE, IN ITS INTERMEDIATE FORMS. 



contains in its substance coloured spots ; these, when 

 minutely examined, are found to be 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 no 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, 885. 



