8 THE LIFE OF THE MOLLUSCA 
extreme value in tracing relationships of the various 
parts. Its principal elements (Plate IJI., Fig. 3) 
comprise a series of paired nerve centres or ganglia ; 
of these one pair, the “‘ cerebral ganglia,” lying above 
the cesophagus, sends off nerves to (“ innervates”’) the 
head, eyes, and the special organs of sense; another 
pair, the “pedal ganglia,” is situated below the 
cesophagus ; 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 miore 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 
cesophageal 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 
