27 



five per cent, formalin it still retains its fresh appearance 

 and consistency. It can be drawn out into long threads 

 of surprising length (some feet) without breaking. The 

 secretion of this matter takes place very rapidly when 

 the animal is severely stimulated, particularly with 

 irritating fluids. Concerning its function but little is 

 known. It would appear in the first place to be a pro- 

 tective — or defensive — secretion. It does not leave the 

 animal gradually after secretion but is produced, as we 

 have seen, spasmodically and quickly, and raises itself 

 in a sheet. Obviously it removes in this way any dirt, 

 sand grains or other matter from the organs of the pallia I 

 cavity, and, moreover, protects them to a certain extent 

 from the entrance of such materials. 



Histology of Mucous Gland. 



Sections taken through the mucous gland show that 

 we have to deal with a very much modified epithelial 

 layer, which rests on a basement membrane of connective 

 tissue overlying the peculiar cellular connective tissue of 

 the mantle (fig. 40, Con.). This latter tissue extends 

 into the folds, but only occurs as bridges running across 

 at intervals and leaving great cavities between the two 

 sheets of epithelium. The epithelium is composed of 

 three types of cells as described by Bernard: 1, Mucous 

 cells; 2, ciliated cells; 3, neuro-epithelial cells. 



In sections, the characteristic appearance is to find 

 cell walls running from the periphery to the basement 

 membrane dividing the whole up into large chambers 

 filled with mucus (fig. 40, Mu. c), but in addition there 

 is a more protoplasmic peripheral region bearing cilia 

 and another series of cell walls. There are also two 

 distinct nuclear regions, one of which is peripheral 



