422 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
These sense cells are also present on the mantle edge 
proper, in the epithelium covering the adductor muscle in 
the neighbourhood of the visceral ganglia and osphradia, 
on the sides of the gill axis and in the outer epithelium of 
the rectum near the end of the free portion. 
In shape, these sensory cells differ but little from the 
ordinary epithelial cells, which act as supporting cells 
around them. In many cases, however, they are very 
narrow, with the nucleus situated about the middle of 
their length. The narrow cells have the end towards the 
surface of the epithelium swollen out into a disc, which is 
just as broad as the ends of the other epithelial cells 
around. Another feature is that the sense cells stain more 
intensely when a nerve stain is used. The cells are 
provided with a bundle of extremely long cilia, as long, 
or longer, than the cells themselves. These cilia are 
quite characteristic structures, and much longer than 
those of ordinary epithelium. The margin of the sense 
cells shows a distinct striation vertical to the surface, 
as if the sense hairs or cilia were produced into the 
cell, and this striation can often be observed, though less 
distinctly (still deeper in the cell), as a series of lines 
converging towards the nucleus. The base of the cell is 
connected with the nerve fibres, ramifying under the 
epidermal layer and in connection with the deeper lying 
nerves, one of which runs up the centre of each of the 
extensible sensory tentacles. These cells are probably 
olfactory as well as tactile in function. 
(b) Tue Orocysts.—A pair of Otocysts (fig. 27, ot.) 
are present in both P. maaimus and P. opercularis. “They 
are situated quite external to the foot, embedded in the 
visceral mass amidst the connective tissue of the digestive 
gland; and lie beneath the cerebral and pedal ganglia and 
connectives, that is, on the side turned away from 
