OCT. — dec. 1857.] The Pearl Oyster of Ceylon. 



93 



the shell does not grow rapidly, and the mother-of-pearl lining ia 

 jagged at the edge, and is not of the usual brilliant colour. How- 

 ever, its temporary retraction facilitates the ingress of sand and 

 -other irritating particles, which doubtless become the nuclei of 

 many pearls, as will be hereafter observed. The forepart of the 

 mantle is coloured and rayed like the shell. The colouring matter 

 is secreted by glands found in these parts. The glandular secre- 

 tion serves the purpose of increasing the lateral and longitudinal 

 dimensions of the shell. It is after this is deposited, that the 

 pearly secretion (nacre) is applied to the inner wall of the shell, 

 which, concreting or solidifying, increases its thickness. The 

 pearly fluid is secreted by nearly the whole external surface of 

 the mantle. It will be thus clearly understood, that when a grain 

 of sand or the larva of an insect is introduced between the mantle 

 and shell, it will become covered over with the pearly secretion ; 

 which always going on, is augmented at the part where the foreign 

 matter lies. This phenomenon I have detected with the aid of the 

 Microscope in the very earliest stage. 



About one and a -quarter inch from the rim of the shell, is seen 

 a pair of gills like four segments of a circle, or semilunar combs, 

 stretching transversely from one side to the other, the convexity 

 looking forwards. There is a vacant space between the concave 

 surface of the gills and the body of the Oyster. The adductor mus- 

 cle, called " grizzle" is now seen, covered over with a delicate 

 membrane. This muscle is attached to the inner surface of both 

 shells. On one side (the left, when the Oyster is placed with the 

 hinge next the observer) is seen a short, conical, tubular, sharp- 

 pointed prolongation ; this is the terminal end of the intestines ; 

 it looks like a sharp-pointed claw. The intestine is short : leav- 

 ing the stomach, it winds round the adductor muscle, and termin- 

 ates, as I have just remarked, on the side opposite to where the 

 mouth is placed. There is always an unclosed space, between the 

 edges of the mantle, when the tentacles are brought together, ad- 

 mitting of the free passage of excrementitious matter ; and it was 

 through the same opening between the mantle, that I observed, on 

 one occasion, the ova escape, in a cloudy stream, which continued 

 to pass into the water for nearly 15 minutes. I failed to detect the 



