228 THE BIOLOGY OF THE SEA-SHORE 



by a series of transverse commissures. In view of the 

 abundance of communication between cords and the massing 

 of the ganglia in the head region, one seems safe in assuming 

 that the nervous organisation of molluscs is both well 

 co-ordinated and subject to a fair degree of control. 



The sense organs of molluscs show many features of 

 interest. Sensory cells occur scattered over the whole 

 surface of the body, which is thus everywhere sensitive. 

 These sensory cells are most numerous in the most exposed 

 parts such as the head tentacles of snails, the epipodial 

 tentacles of Trochus,Haliotts, etc., and the pallial tentacles of 

 bivalves, which are thus essentially tactile organs (Pelseneer, 

 1906). 



Situated at the entrance to the mantle cavity in nearly 

 all marine molluscs is an organ of smell or chemical sense 

 called an osphradium, the function of which is, probably, 

 to test the water before it passes over the gills. The osphra- 

 dium has the form of a ridge or pit lined with epithelium 

 and is said to be most highly specialised in the carnivorous 

 Gasteropods, particularly the whelk {Bucciniim undattim), 

 in which species it is so large as to resemble a gill. 



Otocysts occur in nearly all molluscs and have, broadly 

 speaking, the same essential structure and functions as those 

 of Crustacea, except that they are more often closed than 

 open. 



Visual organs in the molluscs show considerable diversity 

 of structure. True cephalic eyes are characteristic of 

 Gasteropods ; in nearly all bivalves, where the head region is 

 hardly developed, they are replaced by analogous organs 

 situated on the mantle. Eyes are absent from many marine 

 forms which creep about wet sand, e.g. Bulla, Sigaretus, 

 Scaphander, Philine. In certain species, the eyes, though 

 present, are covered by a layer of skin, e.g. Doris and Natica. 

 The cephalic eyes of Gasteropods vary greatly in the degree 

 of their development. They are least developed in the 

 limpet, where they are little more than pits in the skin lined 

 by pigmented and sensory cells. 



Pallial eyes show similar variation in their degree of 



