0.3 



Millimeters 



Figure 101. — Photograph of crystals of a mixture of calcite and gypsum formed in the mantle cavity of C. virginica. 



chemical reactions show that the secretion of 

 calcium is not confined to special sites but takes 

 place over the entire edge and outer surface of 

 the mantle. The intensive coloration of the 

 granules by alizarin suggests that they contain 

 a considerable amount of calcium, probably 

 bound in organic compounds of the globules. 

 Amoebocytes present in the material secreted 

 by the mantle also may be involved in the mobili- 

 zation of calcium during the formation or repair 

 of shells. 



Sometimes the mineral crystals formed by the 



mantle are not incorporated in the conchiolin but 

 accumulate in the pallial cavity and are eventually 

 ejected. On several occasions fairly large quanti- 

 ties of a wliite powdered material were found in 

 front of the discliarge areas of oysters which were 

 ke])t iti glass trays in running sea water in the 

 laboratory. The material consisted of crystals 

 (fig. 101) wliich, according to the X-ray analysis 

 kindly performed by Marie Lindberg of the 

 Geochemistry and Petrology Branch of the 

 Geological Survey of the U.S. Department of the 

 Interior, were found to consist of a mixture of 



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FISH AND WILDLIFE SERVICE 



