The Tinnevelly Pearl Banks. 423 



of the groove in the foot, is thus left, attached to the base of 

 the foot at one end, and to the rock at the other. The process 

 is again and again repeated until a strong cable is formed ■ and 

 it was one of the most important results of the careful investi- 

 gations of Dr. Kelaart in Ceylon, that the power of the animal 

 to cast off its byssus at pleasure was ascertained. It leaves it 

 behind to make another in a more convenient place, like a ship 

 slipping her cable and going to sea. From this ability to shift 

 its berth it follows that the pearl oyster might safely be 

 taken from its native beds, and made to colonize other parts of 

 the sea ; and also that it would move of its own accord if the sur- 

 rounding water should become impure or sandy, or when there 

 is an influx of fresh water. The animal can re-form the byssus 

 at pleasure, if in good health and condition. 



The formation of pearls is another point which has received 

 much attention, but which has not as yet been definitively 

 settled. Pliny and Dioscorides believed that pearls were pro- 

 ductions of dew, but that obseivant old Elizabethan navigator, 

 Sir Richard Hawkins, shrewdly remarked that "this must be 

 some old philosopher's conceit, for it cannot be made probable 

 how the dew should come into the oyster." Modern investi- 

 gation has suggested various causes for the intrusion of the 

 nucleus round which the pearl is formed. The free border of 

 mantle lining each valve of the shell dips downwards to meet a 

 similar edge on the opposite side, thus forming a double fringed 

 veil. The tentacles of this fringe consist of long and short flat 

 filaments, which are exceedingly sensitive, so that even the ap- 

 proach of a foreign substance makes them draw forwards and 

 shut out the intruder. They doubtless prevent the pearls from 

 dropping out of the shell, and preserve the fish from the host of 

 carnivorous creatures which infest its place of abode ; and if it 

 be true that particles of sand form the nuclei of pearls, they 

 must run the gauntlet of these ever-watchful sentinels before 

 they can intrude themselves amongst the interstices of the 

 mantle. The food of pearl oysters consists of foraminifera, 

 minute algae, and diatoms ; and Dr. Kelaart has suggested that 

 the siliceous internal skeletons of these microscopic diatoms 

 may possibly permeate the coats of the mantle, and become 

 nuclei of pearls. 



Lastly, the ova which escape through the distended coat 

 of an overgrown ovarium may, perhaps, become embedded in 

 the interstices of the mantle, and become the nuclei of pearls, 

 especially as pearls are usually found embedded in the mantle 

 near the hinge, where the ovarium is most liable to rupture. 

 Large pearls often work their way out of the mantle, and 

 lie loose between it and the shell, or become attached to 

 the surface of the latter. They have even been found outside 



