82 



deeper-staining contents. Bernard is in error in saying 

 they possess no pigment. 



Underlying the epithelium of the osphradial leaflets 

 is a delicate hasement membrane and a connective tissue 

 layer with prominent muscle fibres. The centre of the 

 leaflet is occupied by irregular blood spaces, which com- 

 municate with a very definite sinus running down the 

 external margin. 



The nerves of the osphradial leaflets are derived from 

 a central ganglion, situated along the axis of the organ. 

 This ganglion consists of a central mass of fibrillae, the 

 neuropil, with the usual cortex of ganglion cells. The 

 ganglion cells occur, therefore, on all sides, but are, 

 nevertheless, more concentrated laterally, except where 

 nerves are given off to the leaflets. 



To each leaflet a nerve is given off, and this leaves 

 the side of the ganglion exactly halfway down. This 

 principal leaflet nerve runs outwards across the leaflet, 

 nearer the inferior border than the opposite and attached 

 edge (fig. 44, Os. n.). It gives off numerous branches on 

 both sides, but particularly on that towards the mantle. 

 These latter branches are very regularly arranged, and 

 from them arise the neuro-fibrillae which enter the 

 epithelium to form the nerve-net already described. 



The Otocysts. 



The otocysts are spherical sacs formed of a delicate 

 epithelium. The cells are low and irregular in shape 

 with but little contents. Two types are present, sense 

 cells and non-sensory cells. 



The depth of cells is only ^ of the diameter of the 

 otocyst. The large cavity is filled with calcareous matter 

 forming an otolith. 



