TTPE MOLLUSC A. 



281 



ance with the absence of metamerism, lacks the ladder-like 

 arrangement which characterizes the Annelida. Nevertheless 

 there are two ganglionic masses, each in typical cases com- 

 posed of two ganglia which may be homologized with the su- 

 praoesophageal and the most anterior suboesophageal ganglia 

 of the metameric forms, and are known respectively as the 

 cerebral (Fig. 124, ce) and pedal (pe) ganglia. The former lies 

 above the oesophagus behind the buccal 

 mass and is connected by nerve-cords 

 termed connectives, surrounding the oeso- 

 phagus, with the pedal ganglion. The 

 cerebral ganglion gives off nerves which 

 pass to the eyes and otocysts (ot) and to 

 the tentacular structures of the head, 

 while the pedal ganglion receives its 

 name from the fact that it sends nerves 

 to the muscular mass forming the foot. 

 In addition to this system of nerves and 

 ganglia there is another system highly 

 developed in the Mollusca which would 

 seem to correspond to the visceral system 

 found in some other forms. It consists 

 typically of a pair of pleural ganglia (pi), 

 one of which lies upon either side of the 

 pharynx, being united by connectives 

 with both the cerebral and pedal ganglia. 

 Prom each pleural ganglion a nerve-cord 

 passes backwards to unite with one or 

 more visceral ganglia (vi), situated below the intestine near its 

 posterior termination, and on each of these visceral cords a 

 ganglion occurs, the parietal ganglion (pa), from which nerves 

 pass to the gills, or rather to the sense-organ which is in con- 

 nection with them. The pleural ganglia innervate especially 

 the mantle and the body-wall behind the head, the visceral 

 ganglia send branches principally to the various viscera, while 

 the parietal ganglia, in addition to the branches which go to 

 the gills and their sense-organs, also assist in the innervation 

 of the mantle. 



Besides these principal ganglia, however, others connected 



Fig. 134.— Diagram of 

 Nervous System of 

 mollttsk. 

 bu = buccal ganglia. 



ce = cerebral ganglion. 



ot = otocyst. 

 pa = parietal ganglion. 

 pe = pedal ganglion. 



pi = pleural ganglion. 



m = visceral ganglion. 



