VII MOLLUSGA—THE SENSORY ORGANS 163 



epithelial lining of the mantle cavity. Here, as in other parts of the 

 body, three kinds of epithelial cells can be distinguished : (1) undiffer- 

 entiated cells, which may contain pigment, and are usually ciliated ; 

 (2) glandular cells ; (3) sensory cells. The proportions in which 

 these three kinds of cells appear varies in different regions of the 

 mantle. If glandular cells prevail on a certain area, that area assumes 

 a glandular character, and may even develop into a sharply localised 

 epithelial gland (e.g. the hypobranchial gland). On the gills, undiffer- 

 entiated ciliated cells predominate. Where sensory cells predominate 

 a sensory character is given to the region ; such a region, if sharply 

 circumscribed, the sensory cells continually increasing in number, 

 becomes a pallial sensory organ. The gradual development and con- 

 tinuous differentiation of such an organ may be particularly well 

 traced in the Frosobranchia, the sensory organ developed being the 

 osphradium. In consequence of its position in the mantle cavity, and 

 especially on account of its proximity to the gill, it has been assumed 

 that its principal function is ' to test the condition of the respiratory 

 water, or, in other words, that it is an olfactory organ. 



The osphradium among the Prosobranchia is least dift'ereiitiated in the Dioto- 

 cardia. In the Fissurellidce it does not exist as a sharply localised organ. In the 

 Monotocardia it becomes more and more differentiated, and has a special ganglion, 

 and finally in the Toxiglossa, it reaches the maximum of its development. 



A review of the jjosition and number of the osphradia has already been given in 

 another place (§ V. p. 71). As an example of the special form and structure of this 

 organ we select the highly developed osphradium of a Toxiglossa, Cassidaria 

 tyrrhena. 



The osphradium of Cassidaria is a long organ, pointed at both ends, which lies 

 to the left of the ctenidium on the mantle in the mantle cavity. As in other highly 

 specialised Monotocardia (Fig. 71, p. 73) it looks like a gill feathered on both sides, and 

 has on that account been regarded and described as an accessory gill. It consists of 

 a ridge rising from the mantle, which in transverse section is almost square, and 

 carries on each side 125 to 150 flat leaflets, which stand at right angles to the 

 surface of the mantle, and are so closely crowded that their surfaces are in contact. 

 The ridge consists almost exclusively of the long osphradial ganglion. Each leaflet 

 receives from this ganglion a special nerve, which runs along its lower projecting 

 edge, and sends off four principal branches into it. In its dorsal pallial side each 

 leaflet contains blood sinuses, which communicate with a sinus lying above the 

 ganglion in the ridge. 



These principal nerves in the leaflets branch, and their last and finest ramifica- 

 tions penetrate the supporting membrane between the epithelium and the sub- 

 epithelial tissues. These become connected with the branches of the interepithelial 

 ganglion cells, each of which again is connected with a si)indle-shaped epithelial 

 sensory cell. The branched interepithelial cells are connected together by their 

 processes. 



The sensory epithelium above described is developed on the lower surfaces of the 

 osphradial leaflets, i.e. those turned to the mantle cavity, the indifferent, non-ciliated 

 cells on these surfaces being filled with granules of yellow pigment, while in the upper 

 surfaces of the leaflets these cells are devoid of pigment and ciliated. Glandular 

 cells are also found definitely arranged in the epithelium of the osphradial leailets. 



