ICJ22] MADRAS PEARL FISHERIES 119 



If we watch a living pearl oyster in an aquarium tank, we see 

 that in the normal condition it lies with the valves slightly agape, 

 their edges fringed with a feathery frilling along each side of the 

 opening. The oyster seldom opens more than %th of an inch ; 

 but this is ample to permit of breathing and feeding. Like the 

 common sea mussel, the pearl oyster is attached throughout life to 

 some solid object; when much crowded they adhere to one another 

 in clusters. 



The anatomy is easily understood if we remove one valve, say 

 the right, together with one gill and the thin lining of tissue 

 adhering to the inner surface of the valve. This delicate layer is 

 one-half of the mantle or pallium and is in reality a simple fold of 

 the body wall of the animal. With its companion fold on the 

 other side, it envelops the whole visceral mass of the animal much 

 as the flaps of a coat enwrap the body of the wearer. The external 

 surface of each mantle fold is attached lightly to the inner surface 

 of the shell, and is equal to it in extent when in the uninjured and 

 unretracted condition. Its edge is thickened and reflected inwards 

 into a second edge. The cells of the true edge secrete the horny 

 periostracum, those of the reflected one are mainly tactile, the most 

 sensitive being borne on delicate dendritic processes that lightly 

 interlock with those of the opposing mantle edge, thus forming a 

 most sensitive cheveux-de-fnse guarding the entrance to the cavity 

 enclosed between the mantle folds — the pallial cavity. 



The mantle itself consists of a thin matrix of loose connective 

 tissue containing blood spaces, muscle fibres arranged fan-wise for 

 the retraction of the mantle edge, and a fine network of nerve fibres. 

 Externally where it is in contact with the inner surface of the 

 shell, it is covered with a layer of brick-shaped secreting cells, the 

 external pallial epithelium. These normally secrete the mixture 

 of carbonate of lime and shell gelatin (conchyolin) which crys- 

 tallizes into nacre after it is excreted. But it is notable that this 

 same epithelium that normally secretes nacre, possesses the 

 power to secrete all the other layers if need demands. This I 

 proved experimentally both by injuring the edge of the shell and 

 by removing pieces from the centre of the valve, exposing the 

 ectoderm. In both cases, a layer of periostracum was quickly 

 formed ; only after an interval of several days was the secretion of 

 nacre resumed. 



