By means of Mallory''s triple staining method, it was pointed out that the mantle of the pearl oyster (Pinctada martenii Dunker) contains 4 kinds of secretory cells, such as mucous cells, cells with large secretory granules, cells having both mucous and large secretory granules, and cells including characteristic brown granules. Rabl''s method made it possible to demonstrate total Ca content. In the demonstration of insoluble Ca by means of the production of laque with metallic elements, the author was able to obtain a sufficient result by applying Cretin''s method. So far as the relation between Ca content and the morphology of glandular cells is concerned, there is no special gland which is proper to a calcareus function.