Mother -of -Pear I and its Uses. 389 



ing from the roof, linked together after the manner of a 

 chain, or clustered in large piles, firmly attached to one 

 another. This attachment is only temporary. It has been 

 generally believed that the pearl oyster is a fixture, and 

 certainly the appearance of the cable by which it binds 

 itself to the rock would warrant that supposition. This 

 attachment has the look of a large tassel, consisting of an 

 infinite number of slender filaments, each about the thick- 

 ness of a packthread. It springs from the body of the 

 mollusc, and passes through an orifice between the shells, 

 immediately next the hinge. During life its colour is 

 iridescent, changing from a dark green to a golden bronze, 

 exhibiting while in motion various prismatic hues. It 

 fastens itself to the rugged rocks with so determined a hold 

 as frequently to require the utmost strength of a powerful 

 man to tear it from them. Under these circumstances it 

 seems incredible that the mollusc should move from place 

 to place. But to borrow the words of Galileo, " Neverthe- 

 less, it does move ; ' and under the influences of certain 

 causes, these bivalves are in the habit of migrating en masse, 

 not for any great distance, it is true, yet from one coral 

 shelf to others in the immediate neighbourhood. As con- 

 cerns the reason of their exodus, it may possibly be 

 an alteration in the temperature of the water, caused by 

 a change of weather, or a scarcity of the animalculae upon 

 which the oyster feeds. 



The presence of drift-sand is obnoxious to its com- 

 fort ; consequently, in the neighbourhood of banks and 

 crags composed of that kind of debris it will not live. 

 In lagoons which have no tideway it is not found, and 

 if introduced there, perishes. The experiment has fre- 

 quently been tried, and its failure seems traceable to 

 the following cause : Wheresoever sea-water becomes 



