356 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
together with a fold of this layer with a slight support of 
the underlying connective tissue (fig. 6, P.gr.). The 
periostracum (fig. 6, ?.) 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 the 
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 inner 
surface of the mantle lobes is also ciliated. 
Insinuated between the ordinary epidermal cells on 
the outer surface of the mantle, near the margin are to be 
seen peculiar cells (fig. 5, Hos.) 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 Herdman 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 circumpalhal nerve 
