IV] LIFE-HISTORY AND ENVIRONMEN^T 45 



described. Holding on by these strands the animal 

 pulls itself nearer to the point of attachment, then 

 stretches out its foot and proceeds to secrete other 

 byssal threads and to attach them further away, on 

 the glass sides of the vessel. As soon as a firm hold 

 is obtained the old byssal threads are thrown off by 

 the animal and remain adhering to the glass. This 

 process is repeated time after time, and the track 

 followed by the mollusc is indicated by the series of 

 cast-ofF byssal threads. 



Apparently the young pearl oysters do not travel 

 so fast as young scallops (which as a matter of fact 

 are famous throughout life for their powers of 

 locomotion). They are sensitive to light and, in 

 aquaria, move but little during the day. Probably 

 this does not apply at the bottom of the sea on the 

 pearl banks. During the first two years of life the 

 pearl oyster grows rather quickly. The age and 

 the size at which they are fishable, vary for the 

 different species. The Ceylon pearl oyster has to 

 battle with its enemies until it is four and a half 

 to five years old. It is then roughly about three 

 inches in height and two and a half inches in breadth. 



The life-history of the fresh-water mussels in 

 rivers and lakes is very different. The mussels live 

 in clear water and on stony or sandy ground, either 

 isolated or in great numbers. 



They do not emit eggs outside their shell, for 



