R4 



sense orcan is really verj' near to and for purposes of 

 sensation, practically at the surface. 



The Osphradia. - These J,lolluscan orr;ans of special sense 

 fonn large masses of complex tissues at either side of the 

 visceral ganglion. (fig. 60) . Their general shape is ellip- 

 tical and they are in close association frith the branchial 



nerves. Each organ (fig. 61) is composed of tvro parts. 

 At the ventral (outer) surface, there is a part of the 

 body epithelium, v/hich in this region is specially differ- 

 entiated from the surrounding cells. Y/hile the epitheliiim 

 of the epibranchial cavity is ciliated, the osphradial ep- 

 itheliiom is quite devoid of cilia. Besides, the cells 

 composing the osphradial epithelium seem to liave quite 

 lost their cell-walls, so that the spherical muclei lie in 

 a common mass of protoplasm. The outer surface of the 

 epithelial layer is covered by a very delicate membrane, 

 and at its internal surface there is a stouter basal mem- 

 brane. Underlying the surface epithelium is a mass of 

 nervous elements, composed of both cells and nerve fibres. 



The cells, hov;ever, are sensory and stain somewhat dif- 

 ferently from the ordinary ganglion cells. These sensory 



