56 COMPARATIVE ANATOMY 



further, as two rims bordering a narrow groove. Immediately 

 below the point of fusion the edge of each mantle-flap is 

 thickened and pigmented, and slightly scolloped out, and below 

 this point again the thickened edges are slightly scolloped out 

 and bear a fringe of small tentacles. In this manner two slit- 

 like passages are left between the opposed edges of the mantle. 

 The lower passage, bordered by tentacles, is the inhalant, 

 the upper one is the exhalant aperture. 



The outer and middle layers of the shell are secreted by the 

 epithelium of the thickened border of the mantle. The inner 

 nacreous layer is secreted by the whole of the outer surface of 

 the mantle. On cutting away the mantle-flap of one side, the 

 general shape of the animal and some of its more important 

 organs lying in the mantle-cavity are exposed. Ventrally and 

 anteriorly is the triangular muscular foot, its anterior margin 



