52 PEARLS AND PEARLING 



the border of the shell. In the living mussel the mantle 

 is joined to this line and to the muscles at the upper part 

 of the shell. 



A living mussel can be opened by inserting a thin 

 knife and cutting toward the hinge at both ends. 



Illustration No. 5 shows a Quadrula heros, or "wash- 

 board" mussel exposed. A very soft thin tissue called 

 the mantle covers the entire inside of both shells, being 

 joined together at the upper part of the shell, and is fas- 

 tened to the shell at the pallial line and to muscles in the 

 upper part. 



The edge and outside of the mantle produces a sort 

 of milky liquid, containing carbonate of lime and animal 

 matter, which builds up the shell by thickening it from 

 the inside and by adding to it at the edge, the pearly 

 nacre near it, a layer of vertical prismatic cells outside 

 this and over all, the protecting conchiolin epidermis. 

 Ordinarily the growth of the shell is only such as to 

 meet the needs of the growing animal and for its pro- 

 tection. After the mussel reaches maturity, the shell 

 ceases to grow, excepting in some varieties which con- 

 tinue to lay on nacre only. The mantle is carried be- 

 tween the locking hinge teeth and fills the space between 

 them like a cushion. In the illustration the mantle is 

 folded back to show other parts of the animal. A mass 

 of tough white muscular fibers will be found at each end 

 of the shell on the inside near the back. These are the 

 powerful adductor muscles by which "the animal closes 

 the shell. 



The muscles allow the shell to open but a short dis- 



