8 THE LIFE OF THE MOLLUSCA 



extreme value in tracing relationships of the various 

 parts. Its principal elements (Plate III., Fig. 3) 

 comprise a series of paired nerve centres or ganglia ; 

 of these one pair, the " cerebral ganglia," lying above 

 the oesophagus, sends off nerves to (" innervates ") the 

 head, eyes, and the special organs of sense ; another 

 pair, the " pedal ganglia," is situated below the 

 oesophagus ; whilst the " pleural ganglia,'' lying one 

 on each side just above the pedal ganglia, form the 

 third pair. These several ganglia are united by 

 nerve cords, so that the whole forms a ring or collar 

 round the throat. In the more primitive Mollusca 

 the two last named are somewhat removed back 

 from the other pair, but in the more specialized 

 forms they are in proximity to it, and the resultant 

 oesophageal ring is much more concentrated. Four 

 nerve cords run back from either side of the ring, 

 two in connection with the pedal ganglia serving the 

 foot, and two in connection with the pleural ganglia 

 innervating the viscera. These last, which are the 

 more important, and are provided with minor nerve 

 centres, are united towards their terminations, thus 

 forming a continuous loop, known as the " visceral 

 loop." 



Various sense organs are present. Many of these 

 are situated on the integument, and are probably, 

 like the tentacles, organs of touch. Eyes are found 

 in most forms, sometimes very perfect organs, as in 

 the higher Cuttlefishes ; in Snails generally they are 



