254 THE CORAL LANDS OF THE PACIFIC. 



these bivalves are in the habit of migrating en masse from one 

 coral shelf to another in the immediate neighbourhood. This 

 is notably an effect of sudden change of temperature, or a 

 scarcity of the animalcules on which it feeds. 



When pearl-oysters grow singly it has been noticed there arc 

 very few pearls; where crowded together or jammed into 

 crevices of rocks, the reverse is the case. This may have some 

 connection with want of liberty to move about, whereby it is 

 possible they become diseased. However unlikely their 

 migratory powers may appear from the aspect of the shell and 

 the apparently immovable manner in which they attach them- 

 selves to the stone, I will give a proof, on first-class authority, 

 which all experienced fishers will recognise as conclusive. 

 Young pearl-oysters are usually found in vast multitudes 

 packed closely together. Several bushels of them Avill fre- 

 quently be attached to a single stone, filling up all hollows in 

 a compact mass. It is perfectly evident that they cannot con- 

 tinue to grow that way, but as they increase in size they must 

 loosen themselves and migrate elsewhere. It is certain that an 

 oyster the size of a sixpence is as firmly bound to the stone, in 

 proportion to its strength, as is one the size of a soup-plate ; 

 and if the small ones have the power to move, so have the 

 large ones. 



I have long been of opinion that the pearl-oyster of the coral 

 lagoons is not spawned altogether within the lagoon, but chiefly 

 in the deep sea outside, for the reason that if any man will go 

 between the months of December and March (which seems in 

 the Pacific to be the breeding season for many marine crea- 

 tures), and stand upon the outer edge of a flat reef, on the 

 windward side of any pearl lagoon, when the tide is making, 

 he will observe the water to be everywhere full of young 

 pearl-oysters no bigger than his finger-nail, and others much 



