THE TOMB OF PEARL 



structed the shell itself by continually forming cal- 

 careous matter which spread in thin layers of mother- 

 of-pearl one upon another. It was able not only to 

 build its own house, but to keep enlarging it all the 

 time, making it continually thicker, adding new layers 

 to those which were already there. And, as the valves 

 enlarged, the oyster grew. 



For making the shell, the body makes use of two 

 large symmetrical expansions which envelop it com- 

 pletely like a cloak, and, in fact, the name " mantle " 

 is actually given to them because of their nature. 

 These two parts of the cloak form and lay down, on 

 their outer side, the calcareous matter of the mother- 

 of-pearl of which the shell is made. If we examine a 

 whole oyster when it is alive, we first separate the 

 two halves of the shell and see that the calcareous mass 

 is lined inside by a thin plate of living tissue forming 

 the two expansions which make the cloak. In the 

 spacious cavity which they bound, but occupying only 

 a small space in it, is the body itself with its organs, 

 the most notable of which are those of respiration, 

 branchial plates lodged in the angle between the cloak 

 and the sides. This type of organization is also that 

 of other molluscs of the same class, though of different 

 appearance, the " Lamellibranchs," the ordinary oysters 

 of commerce, mussels, clams, etc., which also have a 

 two-lobed cloak and a shell with two valves surrounding 

 the whole body. 



When alive, in their native waters, the pearl oysters, 

 on the bottom where they live and where the fishermen 

 gather them, leave their shell half open, and their 

 valves allow the water from outside to penetrate freely 

 into the large interior cavity. Two currents are set 

 up, one going in and the other coming out, so that the 

 water is continually renewed, bringing dissolved 

 oxygen to the gills, and particles of food to the digestive 

 tube. The thick, pleated edges of the mantle, which 

 line those of the shell, have fluted contours so arranged 

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