Moihcr-of-Pearl Shell. 91 



heel of the shell, and attaches it to the bottom ; in 

 old shells however this byssus is completely wanting. 



The oyster is usually open an inch or two at 

 the lips, to admit the sea water and any organic 

 substance that the tides and. currents may bring to 

 serve as food. On being disturbed it immediately 

 closes, and it is no uncommon occurrence when the 

 water is thick for a young diver to come up with 

 a shell fast on his fingers : in order to free him a 

 knife must be used to cut the muscle of the oyster, 

 and thus allow the shell to open. After being 

 taken from its natural element, and exposed in a 

 boat to the tropical sun the shell opens, but it shuts 

 tightly again on being touched. The oysters do not 

 generally live longer than twelve hours after being 

 taken from the water, and a dry wind off shore 

 hastens their death ; it is indeed a wonder, that, 

 deriving their oxygen from the water, they can live 

 so long in our atmosphere. 



Mother-of-Pearl shells are taken as low down 

 as 45 and 50 fathoms ; beyond this depth the divers 

 have hitherto been unable to explore, and hence 

 their extreme limit is unknown. One has, how- 

 ever, been brought up, when splicing a cable, from 

 a depth of 150 fathoms. The zones of all sub- 

 marine animal life are regulated by the pressure, 



