MANTLD. 



The inside of each valve of the shell is lined by a thin fold 

 of tissue, a mantle lobe (fig. 10, m.). The mantle lobes* are 

 united to each other along the hinge line and anteriorly and 

 posteriorly for a space that practically corresponds to the width 

 of the ears of the shell. 



W/ith the exception of the free borders of the mantle, each 

 lobe consists of a very thin membrane that is closely applied to 

 the inner surface of the shell which it entirely covers in the 

 living extended animal. In specimens that have been disturbed 

 so the shell valves are closed together, the margins of the mantle 

 lobes are drawn far back into the shell so there may be a strip 

 of three-quarters of an inch or more of the inner border of each 

 shell valve that is left uncovered. This retraction of the mantle 

 is necessary in order that the thickened and highly modified 

 margins of the mantle may not be injured by the closing of the 

 shell. 



Each mantle lobe is free from the rest of the body except 

 where it covers and is fused with the adductor muscle and diges- 

 tive gland, where it is joined by the posterior border of each of 

 the outer labial palps, and where 'it is joined by the membranes 

 that support the gills. 



The formation of the shell is due entirely to the secretion of 

 materials by the mantle. The nacre, which forms the inner 

 surface and most, if not the entire thickness of the shell is 

 secreted by the whole of the applied surface of the mantle and 

 in healthy perfect shells is quite smooth and white. As it is 

 constantly being added to, it is thicker in old than in young 

 shells and in the older part of the shell, toward the middle of 

 the hinge than elsewhere. Certain portions of the mantle, as 

 that along the margins between the pallial lines and the free 

 borders, and in the region of the hinge line and wings, are more 

 active than others in secreting nacre. In these regions the 

 epithelium on the shell side of the mantle is composed of espec- 

 ially large apparently actively secreting cells, while the remain- 

 der of the surface is composed of small less active cells. This 

 arrangement is in accord with the especially thick portion of 

 the shell along the hinge line including the ears, and with the 



12 



