38 



a poison gland in the whelk, opening, as it does, so far 

 back. It is in all probability a digestive gland, but it 

 might function as an "antiseptic." 



The Salivary Glands possess an extremely compact 

 structure. They become brittle in preserved specimens, 

 and are most difficult to cut when embedded in paraffin. 



Sections show that after the salivary ducts break up 

 in the gland, the branches dividing into finer tubules in 

 their turn soon end through complete occlusion of their 

 cavities. Sections of the ducts with their columnar cells 

 bearing cilia (fig. 26, Sal. d.) are met at irregular 

 intervals, but the greater part of the tissue is composed 

 of groups of large intensely vacuolated cells (fig. 26, 

 S. gl. c). These large cells become continuous with the 

 epithelium of the ducts, but, unlike the gastric gland 

 cells, they are so large that the cavity of the ductule soon 

 ceases to exist when the walls are formed of them. In 

 most of the cells a trace of protoplasmic contents at least 

 remains, and there is a distinct nucleus. The rest of the 

 cell is either one large vacuole with colourless non- 

 staining contents, or is filled with bodies of irregular 

 shape and very variable size, which stain intensely with 

 eosin (fig. 26, S. gl. c'). 



In some gastropods the secretion of the salivary 

 glands contains a large amount of free sulphuric acid, 

 which is supposed to aid in the boring of calcareous 

 shells. No marked acid reaction could be obtained with 

 the whelk secretion. Griffiths finds that the gland has 

 the same function as that of Patella, and contains a 

 ferment capable of transforming starch into glucose. 



The Oesophagus. — The section figured has been cut 

 not far from the caecum (PI. Ill, fig 25). It has already 

 been mentioned that longitudinal ridges run along the 

 lumen of the oesophagus. These can often be seen from 



