21 PEARLS [CH. 



called the Hypostracum, and is, like the two preceding 

 layers, formed of calcium carbonate. It is built up 

 of distinct columns arranged at right-angles to the 

 surface of the shell. 



Now let us briefly look at the course of shell 

 growth. The shell grows in size at the edge. Here 

 the edge of the mantle is forming periostracum and 

 prismatic layers, and there is practically nothing else. 

 A little distance in from the edge of the shell the 

 surface of the mantle is secreting nacreous or mother- 

 of-pearl substance, and this goes on continually. 

 Thus the shell increases in thickness over its whole 

 inner surface. 



The secretion of the shell is really very compli- 

 cated, though the organs forming it may appear very 

 simple. Shell formation is due entirely to the cells 

 covering the mantle. These cells fabricate and pour 

 out (or secrete) a mixture of organic and inorganic 

 substances (conchyolin and lime). Crystallisation of 

 these substances takes place altogether outside the 

 animal and the shell is the product. 



What determines the regularity of structure or 

 the formation of beautifully marked or sculptured 

 shells is unknown. All the cells seem practically 

 alike, yet there must be some determining factor 

 hidden there. If a mollusc shell is broken (or a 

 piece of the shell is removed) some distance away 

 from the edge, a most extraordinary thing happens 



