42 



branches, the following changes are to be noticed in the 

 character of the cells. The walls are at first made up of 

 ciliated columnar cells (fig. 29, Dg. cJ), which resemble 

 those of the stomach. The protoplasm is distinctly seen 

 here, since there is little or no secreted or absorbed 

 substance. These cells are gradually replaced by more 

 and more vacuolated ones (fig. 29, Dg. c"), containing 

 the intensely staining bodies. Cilia disappear, and 

 finally the typical broad cells almost meet in the centre, 

 so that the cavity becomes very narrow, almost occluded, 

 as the end of a caecum is reached. 



There are usually said to be three kinds of cells 

 present in the digestive gland of molluscs — ferment 

 cells, granular cells (liver cells), and lime cells. It is 

 extremely difficult to make out these different types in 

 the whelk. Very occasionally cells are met with which 

 are possibly lime cells, but most cells are alike in con- 

 taining the oily or fatty bodies already referred .to. 

 Whether it is really possible to draw a distinction 

 between ferment cells and granular cells is very doubtful. 

 It is probable that both are merely stages in the life of 

 the same cell. 



Intestine.— The intestine differs hardly at all in 

 structure from the oesophagus. There is the same layer 

 of very tall and narrow epithelial cells, with long cilia. 

 Gland cells, however, are far more numerous. The 

 eosinophilous cells are present in considerable numbers as 

 before. The lumen contains large quantities of the 

 granules from the latter cells, and considerable numbers 

 of extruded cells or parts of cells. 



