PATELLA. 211 



covering the attachment of the pallial muscle is com- 

 pressed and pigmented, and that covering the internal 

 pallial muscle between the tips of the shell muscle is 

 compressed but not pigmented. The epithelium clothing 

 the under side of the mantle skirt is also columnar, folded 

 to a less extent, and not pigmented. The ventral 

 epithelium of the nuchal roof is shortly columnar. Many 

 of the epithelial cells of both the dorsal and the ventral 

 epithelium, are clear and glandular, and near the edge of 

 the mantle skirt a great deal of sub-epithelial glandular 

 tissue is present, one set of glands opening at the extreme 

 edge of the mantle and the others further vent-rally. 

 These glands secrete the material for the outer shell 

 layers. Many sense cells are present in the epithelium 

 of the mantle and of its tentacles, which are best described 

 in a separate paragraph. These are particularly abundant 

 in the epithelium of the dorsal side just near the free edge 

 which, as has been stated, is not folded. 



The substance of the mantle, including the roof of the 

 nuchal cavity, is made up of a basis of connective tissue 

 containing numerous blood spaces and traversed by 

 muscles and nerves. The blood spaces in the roof of the 

 nuchal cavity are specially large and are traversed by 

 distinct trabecular of connective tissue. This fact, com- 

 bined witn the comparative thinness of the epithelium, 

 is in itself adequate evidence of the respiratory function 

 of the nuchal roof. The other more regular blood spaces 

 are best considered along with the remainder of the 

 circulatory system and with the pallial gills. 



The chief aggregates of muscle fibres are those forming 

 the pallial muscle and the transverse band which we have 

 called the internal pallial muscle. 



The fibres of the pallial muscle take origin in the 

 pallial impression of the shell and then travel outwards, 



