20 



invertebrate type (fig. 45). Running through this 

 layer are muscle fibres of which the circular are near 

 to the surface whilst longitudinal fibres occur more 

 frequently nearer the central connective tissue mass. 

 The most important feature of the mantle edge is the 

 epithelium, for this is concerned here with shell building. 

 The epithelium covering the free inner surface of the 

 mantle is formed of columnar cells whose height is several 

 times their breadth. These cells are typical ciliated cells, 

 so that the epithelium presents here a ciliated surface. 

 The ciliated cells are separated everywhere by mucous 

 cells. These ciliated cells extend round the edge of the 

 mantle to the shell side. On this side of the mantle (and 

 in that region where the mantle forms the roof of the 

 pallial cavity) there is a remarkable gland running across 

 and opening to the surface not far from the mantle edge. 

 The gland is situated deep below the epithelium imbedded 

 in connective tissue. The actual gland cells communicate 

 with the surface by long processes which are so crowded 

 together that, just below the epithelium and away from 

 the gland cells, they appear like the fibres of a broad 

 nerve. The resemblance is quite striking in methyl-blue- 

 eosin stained sections, for the stain is not unlike a nerve 

 stain. Another very striking feature of the gland is that, 

 instead of the fibre-like communication to the surface 

 opening between epithelial cells, the epithelium appears 

 to be absent for a short space and its place taken, in fact, 

 by the gland cell processes themselves. 



This glandular mass in the Buccinum mantle was 

 noticed by Tullberg, who discusses its function without, 

 however, coming to any definite conclusion. He states 

 that it might very well be a gland for the secretion of the 

 Periostracum, and this is supported by the fact that the 

 gland is absent on the mantle below the visceral mass 



