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one large swelling where the nucleus lies. The position 

 of the nucleus varies considerably, and so it is possible to 

 pack together many more cells than would be the case if 

 the nuclei were all situated at the same level. 



These cells are the indifferent cells of Bernard. One 

 feature that he has overlooked is the well-defined cuticular 

 margin (in sections), crossed by delicate striae arranged 

 perpendicularly to the free surface of the epithelium. 



Now, in addition to these cells, there are, according 

 to Bernard, neuro-epithelial cells which are in connection 

 with the nerve fibres. These neuro-epithelial cells are 

 always fusiform and possess a rounded or oval nucleus. 

 If the osphradium is an important sense organ (and one 

 would assume so from the size of the nerves innervating 

 each leaflet, as well as from the great central ganglion), 

 and if the nerve endings are neuro-epithelial cells we 

 should expect the latter to be numerous and fairly 

 obvious. The contrary is, however, the case, and in 

 ordinary sections it is hardly possible to make out many 

 cells of a different type from the pigment cells already 

 described. 



Are the nerve endings different, then, from those 

 described by Bernard? The osphradium of the gastro- 

 poda appears to be homologous with that of the lamelli- 

 branchiata, and so far as general structure is concerned 

 there is perfect agreement. List has shown that in 

 Mytilus there are free nerve endings in this organ, and 

 the author of this memoir has seen and described the 

 same structures in Pholas and Pecten. Furthermore, 

 Gilchrist has described free nerve endings in Aplysia. 



The most striking similarity in the microscopic 

 structure of the organ in lamellibranchs and the whelk is 

 evident, and the description of the lamellibranch 

 osphradium might be used for that of the whelk. For 



