30 



towards the duets ;['ossess cilia and have smaller 

 nuclei. 



The digestive gland is similarly a great mass of 

 hranchiug tubules imbedded in connective tissue. The 

 cells lining these tubules vary a good deal in size and 

 shape but are very distinctly glandular, and usually so 

 gorged with droplets of a yellowish-green secretion and 

 fine granules that they burst while being ])repared 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 

 Newbigin 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 

 ot the eells. When examined under a higher power the 

 ])igment 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 molluscan 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 occuis in 

 the faeces. 



III. The Odontophore (tigs, (i, 12, 13, 14).— The 

 Odontophore, or rasping organ, characteristic of all 

 ce^ihalous Molluscs, arises in development as a ventral 

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

 with the underlying mesoderm, ultimately gives rise 

 to:— 



(a) A projection on the floor of the bufeal ravity the 

 fHishion (Mesodermic). 



