90 



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 extremel}^ 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 hbres, ramifying under the 

 ej)idermal 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) The Otocysts. — A pair of Otocysts (fig. 27, ot.) 

 are present in both P. maxitnus and P. ojyercularis. 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 



