IV] LIFE-HISTORY AND ENVIRONMENT 49 



climes by the green hydra, and that interesting worm 

 so ably described by Prof. Keeble in one of the 

 volumes of this series. 



The basis of a paar may be said to be those hard 

 bodies to which young and old oysters can attach 

 themselves. This is known as " culch/' and consists 

 of dead corals, lumps of calcareous algae, and old 

 shells of various species. In many places there are 

 suitable banks where the area occupied by sand 

 alone would be available for pearl oyster occupation 

 if " culch " were present. 



Artificial culching, that is, the bringing of suitable 

 material, such as shells, coral blocks or rubble from 

 other places and depositing it on the sandy bottoms, 

 might easily increase the extent of the oyster beds. 

 Somewhat similar methods have been adopted on the 

 coasts of France and Holland, and various structures 

 have been built up for the attachment of young 

 edible oysters or mussels. 



If we consider all the surroundings, solid and 

 liquid, living, dead and inorganic, as making up the 

 environment of the pearl oyster, we find there are 

 favourable and detrimental features. The favourable 

 conditions have been touched upon ; let us now look 

 at the factors which lead to destruction. One of the 

 most important agents in causing the death of the 

 pearl oysters is the shifting of sand. 



This is occasioned by strong currents raised by 



D. 4 



