Microns 



30 



Figure 87. — Small area of mantle showing nerve net. Formalin 5 percent, gold impregnation. Whole mount. Camera 



lucida drawing. 



At the very bottom of the groove the tall epithelial 

 cells are suddenly replaced by short cubical cells 

 (fig. 88, right side) which extend a short distance 

 along the inner side of the groove. 



The material secreted by the periostracal gland 

 accumulates at the bottom of the groove and in 

 the majority of my preparations appears to 

 adhere to the cells of the outer side (left side of 

 figure 88). This, however, is the result of shrink- 

 age caused by dehydration during the processing 

 of sUdes. In preparations mounted in glycerin 

 the conchiolin can be seen in close contact with 

 the epithelium of both sides of the groove. 



The function of the periostracal gland is to 

 supply large quantities of the material required 

 for new shell growth at the edge of the valves. 



The organic matrix (conchiolin) and foliated 

 layers of calcite needed for increasing thickness of 

 the valves, on the other hand, are secreted by 

 the epithelium covering the entire outer surface 

 of the mantle and in close contact with the inner 

 surface of the valve. The epithelium consists of 

 nonciliated cells which are cylindrical near the 

 free margin of the mantle but become flattened 

 and almost cubical in more proximal areas. Both 

 conchiolin-secreting and calcium-secreting cells are 

 present in this epithelium but their cytological 

 differentiation by means of staining reactions or 

 by precipitation of calcium oxalate is not reliable. 

 Mucus cells and oval cells containing eosinophile 

 granules also occur throughout the entu'e surface 

 of the epithelial covering. 



THE MANTLE 



87 



