ANATOMY AND PJITSIOLOGY OF MOLLUSCA 207 



the whole nervous system is characterized Ijy the compactness 

 of its central ganglia. 



Sense-organs. — The sense of touch seems to reside in all 

 parts of the skin, but is best developed in the tentacles. Snails 

 have a delicate chemical sense. Those that hve in water 

 will direct themselves toward a bit of food when a current 

 from the food bathes their bodies. A land-snail will apparentlj' 

 smell food several centimetres awajr. The chemical sense 

 seems to be located either near the opening of the mouth, 

 or, as in some marine snails, in a special organ located on the 

 mantle wall and known as the osphradium (Fig. 196, osph). 

 Eyes are placed in the garden-snail at the tip of the upper 

 tentacles. The eye, which can be retracted into the tentacle, 

 consists of a nearly globular cup containing a central lens and 

 a retina. It is doubtful whether, despite the long-stalked eye, 

 snails can see far ; indeed, they are nocturnal animals and live 

 in dark places during the day, so that they could not make 

 use of a very delicate and perfect optical apparatus. Ej-es 

 occur also in lamellibranchs and cephalopods. They are 

 numerous on the mantle of the scallop, and the squid has a 

 pair of very perfect ej'es. As for hearing, there is little evidence 

 that snails have this sense. The " otocysts " that are found 

 in most gasteropods are probalily (like the otocysts of higher 

 Crustacea) organs of equilibrium. 



