20 PEARLS [CH. 



the mother-of-pearl oysters, but very thin (only about 

 I'o nnn. in thickness), and it is lined by the usual 

 nacreous layer, or mother-of-pearl, which is here 

 exceedingly brilliant and iridescent. The mother- 

 of-pearl layer does not extend to the edge of the 

 valve and consequently the margin is still thinner. 

 It is only composed of two layers, which will be 

 referred to below. The external surface of the shell 

 is rough and dull in colour and usually obscured by 

 various other organisms growing upon it. 



The hinge-line is a ridge which runs along the 

 dorsal edge of each valve. Close to it and extending 

 for about the middle third of its length is a tough 

 black elastic ligament. This ligament, compressed 

 when the two valves are closed, is thus always tending 

 to divaricate the valves and open the shell. It is a 

 dead material and consequently unaffected by the 

 action of the animal. Inside the two valves, however, 

 is a powerful muscle (the adductor, fig. 4) which is 

 attached subcentrally, and this serves to close the 

 shell. The shell valves are thus pulled together by 

 muscular action, but they open automatically when 

 the muscle is relaxed by virtue of the elastic liga- 

 ment. For this reason the shells gape when the 

 animal dies. 



The shell of most molluscs, and practically all 

 bivalves, like the pearl oyster and the fresh-water 

 mussels, is composed of three layers, but a fourth 



