Ill] THE PEARL OYSTER 29 



position for re-attachment, the top of the foot is 

 applied to some rock, and the pit at the end of the 

 byssal groove is thus pressed against the attachment 

 surface. The edges of the groove are now brought 

 together, so that a narrow tube is formed, and down 

 this tube passes a fluid secretion from the byssal 



gland. 



It soon hardens in contact with sea-water, and 

 after about five minutes the foot may be withdrawn 

 from the attachment surface. It will then be seen that 

 an elastic fibre extends from the mouth of the byssus 

 gland pit to the point where the other pit was pressed 

 against the rock. This process is repeated very many 

 times, until a large number of fibres extend in a 

 bunch from the byssus gland opening to the attach- 

 ment surface. 



If a pearl oyster or an edible mussel is pulled 

 away with force from its attachment, the byssus is 

 not usually detached from the foot. It comes away 

 from the rock or it breaks. After some time, how- 

 ever, the mollusc sheds the old byssus, and if it 

 re-attaches itself it does so by the secretion of a new 

 one. 



The Muscles. 



Reference has already been made to the large 

 central adductor muscle which pulls the two valves 

 together and closes the shell. In many of our 



