Ill] THE PEARL OYSTER 33 



— and they occur in the sea, though of course different 

 species are characteristic of all these different 

 situations and are in fact characteristic and different 

 for each season of the year. 



These organisms must be brought within the 

 shell, and as the bivalves are not capable of pursuing 

 them, a current of water is caused to flow into the 

 shell cavity which will bring them with it. This 

 current of water is very important for another reason 

 — the pearl oyster, like other living organisms, must 

 breathe. It obtains its oxygen from the water, and 

 consequently a current of water must enter the shell 

 continually, bringing in oxygen, and must pass out 

 again with carbonic acid gas, the product of re- 

 spiration. 



Thus an inflowing current of sea-water brings 

 oxygen for respiration and food for nutrition at the 

 same time. The water current is produced chiefly 

 by the gills. We have already referred to these 

 organs as being flat plates in the bivalve molluscs. 

 They occur as two double plates on each side, hanging 

 down like curtains from the adductor muscle and 

 visceral mass, and occupying a considerable part of 

 that cavity (itself a part of the external world) which 

 is encompassed by the two valves of the shell. These 

 are really only tivo gills, but as each one appears as 

 two double plates (half gills) there are often said to 

 be four. It must be remembered then that each one 



D. 3 



