80 



towards the ducts possess cilia a'ld liave smaller 

 nuclei. 



The digestive gland is similarly a great mass of 

 hranohing tubules imbedded in connectiYe tissue. The 

 cells lining these tubules vary a good deal in size and 

 shape but are very distinctly glandular, and usually so 

 gorged Avith droplets of a yellowish-green secretion and 

 fine granules that they burst while being prepared for 

 microscopic examination (fig. IT). The ducts of the 

 digestive gland are lined by ciliated columnar epithelium 

 resembling that of the general gut lining. Miss 

 Kewbigin finds that, in sections of specimens hardened 

 in formalin, the epithelium lining the intestine has a 

 band of brownish green pigment near the inner margin 

 of the cells. When examineil under a higher power the 

 pigment is seen to occur in minute closely packed 

 granules, brownish green in mass, green when viewed 

 singly. The cells of the digestive gland vary in size, 

 the large cells near their inner surface contain several 

 of the characteristically mollnscan pigmented vesicles, 

 usually of a brownish yellow colour, while scattered 

 through the protoplasm occur numerous oil drops. The 

 presence of varying amounts of a pigment, called 

 enterochlorophyll, causes great variations in the colour 

 of the digestive gland. The same pigment also occurs in 

 the fieces. 



III. The Odoxtophore (figs. (I, 12, 13, 14).— The 

 Odontophore, or rasping organ, characteristic of all 

 cephalous Molluscs, arises in development as a ventral 

 pouch of the fore gut, the ectoderm of which, together 

 with the underlyinji; mesoderm, ultimately gives rise 

 to:— 



(a) A projection on the floor of the buccal cavity the 

 Cushion (Mesodermic). 



