THE TECHNOLOGIST. [Feb. 1, 1865. 



300- THE TINNEVELLY PEARL-BANKS. 



even the approach 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 car- 

 nivorous 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 algse, 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 over- 

 grown 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 the shell 

 altogether, entangled among the strands of the byssus. When the pearl- 

 banks are under constant supervision, the causes leading to the formation 

 of pearls, as yet imperfectly understood, will, doubtless, receive close 

 attention. 



It now only remains to describe the plan by which it is hoped that, 

 in future, the Tinnevelly pearl-banks will be kept supplied with a 

 sufficient number of well-grown shells to supply a remunerative annual 

 fishery. The idea was suggested by the method adopted with regard to 

 edible oysters on the English and French coasts. The chief external 

 difference between the pearl and edible oyster is, that the former secures 

 itself to rocks and stones by means of byssus, while the latter merely 

 lies flat on the ground on its convex side ; but there is no reason why 

 the pearl oyster should not thrive on artificial banks as well as the 

 edible oyster. 



In the Colne oyster fishery, the brood (oysters two years old) are 

 dredged up out at sea, and placed on " layings " within the river Colne. 

 These layings are about 100 or 150 yards by 80, according to the 

 breadth of the channel, most of them dry at low water, and they are 

 paved with stones, old shells, and any other hard substances, to a depth of a 

 few inches, so as to form a bed for the oysters, which would be choked 

 in soft mud. This material is called " cultch." In France, M. Coste lias 

 adopted a system of placing fascines on the layings, instead of cultch, as 

 a resting-place for the oysters ; but the natural advantages of the ground 

 render any artificial method of tins kind unnecessary in the Colne. It 

 is very important that the cultch should be kept perfectly clear 

 of mud ; above all, that every mussel-shell should be weeded out. These 

 mussels have a remarkable tendency to collect mud round them in heaps, 

 probably owing to their elongated shape, and if they are allowed to re. 



