

S we have just learned in the cases 

 of respiration and circulation, we 

 must not expect to find in the 

 Mollusca orc^ans of sense similar 

 to those of the higher animals. 

 The whole of the body being so 

 soft is no doubt highly sensitive ; nevertheless there 

 are special organs for special senses, and these organs 

 are well furnished with nerves to convey their im- 

 pressions to the local brain. For there is no grand 

 centre of the nervous system as furnished by our 

 brain. Instead of one great trunk-line of nerves and 

 nerve-bundles, as in the vertebrates, we find in the 

 mollusks a system of large and small nerve-threads 

 traversing the length and breadth of the animal, 

 connected here and there by loops {coiii'inissures), 

 and from certain knots (local brains or ganglia) 

 sending off a number of short branches. All these 

 local brains are connected by other nerves, and the 

 chief of them are known as the cerebral, the pedal, and 



