24 



together witli a fold of this hvyer witli a slight support of 

 the uiideiiyiug conuective tissue (hg. 6, P. gr.). The 

 periostracum (lig. 6, P.) emerges from between the two 

 ridges, the cells of which differ from those of the 

 surrounding area. They are glandular, and have deeply 

 staining contents. 



The cells lining the side of the groove nearest to tlie 

 eye bear long cilia, and resemble very closely the sense 

 cells which will be described later. Very short cilia are 

 present on the epidermal cells of the outer margin of the 

 shell fold. The cilia are much better developed on the 

 tip of the ophthalmic fold, which bounds the periostracal 

 groove on the inner side. The epithelium of the iuner 

 surface of the mantle lobes is also ciliated. 



Insinuated between the ordinary epidermal celJs on 

 the outer surface of the mantle, near the margin are to be 

 seen peculiar cells (fig*. 5, Eos.) which contain numerous 

 large rounded granules that stain bright red with eosin 

 or a compound stain containing eosin, such as Mann's 

 methyl blue-eosin. In some places these cells seem to 

 be forcing their way to the surface, and in one or two 

 cases the actual dehiscence of the cell and its contents is 

 observed. They are similar to the cells described as 

 eosinophilous cells by Herdmau and Boyce in the Oyster 

 (42), and in all probability are wandering cells exercising 

 an excretory function. The tentacles of the shell and 

 ophthalmic folds have a similar layer of columnar 

 epithelial cells to those found on the margin of the 

 mantle, but sense cells are particularly numerous at their 

 tips. The connective tissue of the tentacles (containing 

 muscle fibres running longitudinally from the base to the 

 tip) is divided into segments by transverse muscle fibres, 

 which radiate out from the core of the tentacles to the 

 periphery. A branch from the circumpallial nerve 



