PATELLA. 237 



The Pallial Tentacles round the mantle edge have 

 already been mentioned in general terms, and it has been 

 said that they are of two kinds, large and small, of 

 similar structure. A large tentacle can extend as much 

 as \ of an inch beyond the mantle edge. The surface of 

 a tentacle presents a number of fine encircling ridges, 

 and, by the action of muscle strands, it can be retracted 

 into a pit, when its surface is thrown into prominent 

 circular folds. The pits for the large tentacles are 

 situated further ventral than those for the small ones 

 (figs. 8 and 24). The tentacle (fig. 8) is covered by 

 columnar epithelium made up of two kinds of cells 

 resting on a basement membrane : — (a) Relatively broad 

 goblet cells with a basal nucleus ; and (b) Sensory cells 

 which are slender, and narrowest in the middle where the 

 nucleus is situated. The outer end bears a number of 

 stiff sensory processes, so that the cell is of the brush type 

 (pinselzelle) described by Flemming. 



Underneath the basement membrane scattered ganglion 

 cells and nerve fibres can be made out ; the latter converge 

 to the axis of the tentacle, where they form a nerve 

 which is connected with the peripheral part of the nervous 

 network of the mantle. At the points of fusion of the 

 tentacle nerves with this network a few ganglion cells are 

 found. Where a tentacle is retracted its nerve is thrown 

 into a coil. 



Beneath the epithelium of the tentacle is a fairly 

 continuous sheath of longitudinal muscle fibres, the 

 majority of which are inserted into the tip, though some 

 end at the basement membrane. Most of them collect at 

 the base into two bands which run into the mantle in 

 different directions, those of a laterally placed tentacle 

 being relatively more or less anterior and posterior, 

 though not necessarily in the same horizontal plane, Th© 



