THE OYSTER. 17 



which will be explained later on. The food of the 

 oyster consists of invisible organisms which float in 

 the water and are drawn in with it. 



The apparatus for opening and closing the shell is 

 very interesting. If you were to open a check-book, 

 and were to wedge a piece of rubber between the 

 leaves, close to the back, it would form a spring, which 

 would be squeezed by closing the book, and would 

 open it again when released. A book with such a 

 spring would be open at all times, except when forci- 

 bly closed. Wedged in between the two shells of the 

 oyster, at their narrow ends, is an elastic pad, the 

 hinge-ligament, which acts in exactly the same way. 

 When the shell is forcibly closed the ligament is 

 squeezed, and it expands when it is released and thus 

 throws the free edges of the shells apart. The liga- 

 ment is not alive. It is formed, like the shell itself, as 

 an excretion from the living tissues of the oyster, and 

 its action is not under the control of the animal. It 

 keeps the shell open at all times, unless it is counter- 

 acted, and for this reason an oyster at rest and undis- 

 turbed, or a dead oyster, always has its shell open. 



The active work of squeezing the passive ligament 

 and closing the shell is done by a large, powerful 

 muscle, made up of a bundle of contractile fibres which 

 run across the body between the shells, and are fastened 

 to their inner surfaces over the dark-colored spots 

 which are always to be seen on empty oyster shells. 

 The muscle is known to oyster-openers as the heart, 

 and they assure you that when this is cut, the vital 



