VII MOLLUSC A— THE NERVOUS SYSTEM 143 



at once fuses with the cerebropedal, the two entering the pedal 

 ganglion as one connective. Posteriorly, to the right and left of the 

 rectum, near the anus, there are two visceral ganglia of the pleuro- 

 visceral connectives, joined to one another by a commissure running 

 behind the intestine. There are no special parietal ganglia distinct 

 from the visceral or the pleural ganglia. 



There are four buccal ganglia, two behind the gullet or below it (if the intestine 

 is supposed to be horizontal), and two lying laterally and anteriorly to (or above) 

 the muscular mass of the radula. The anterior are connected with the jJosterior, and 

 these to the cerebral ganglia by connectives, and the two posterior and two anterior 

 inter sc by commissures running behind (under) the (esophagus. Nerves run from 

 the posterior buccal ganglia to the small ganglia of a suliradular organ. 



D. Lamellibranehia. 



The nervous system (Fig. 124), like the whole organisation of the 

 Lamellibranehia, is perfectly symmetrical, and consists typically of 

 three pairs of ganglia: (1) the cerebropleural ; (2) the pedal; and 

 (3) the visceroparietal ganglia. These three pairs of ganglia lie, as 

 a rule, far apart, and the connectives uniting them are therefore long. 

 The two pedal ganglia lie close together, while the two cerebropleural 

 and the two visceroparietal ganglia are connected by distinct com- 

 missures beset with ganglion cells. 



1. The cerebropleural ganglia are the result of the fusion of the 

 cerebral with the pleural ganglia. In the Protobranchia, however, the 

 pleural ganglia are still distinct, and lie immediately behind the 

 cerebral ganglia at the commencement of the visceral connectives. In 

 Nucula, the pleuropedal connectives are distinct for some distance, 

 and then unite with the cerebropedal connectives. In Solenomya 

 they still have separate roots, but are otherwise fused along their 

 whole length with the cerebropedal. 



The cerebropleural ganglia are supraoesophageal, and are in 

 contact with the anterior adductor muscle, when this is present. 

 They send nerves into the oral lobes, the anterior adductor, and the 

 mantle. 



2. The pedal ganglia lie at the base of the foot. 



3. The third pair of ganglia, which correspond with the ganglia 

 of the visceral connectives in the Gadropoda, lie posteriorly beneath 

 the rectum, behind the foot, and are generally in contact with the 

 posterior adductor muscle ; in the Frotohrcmchia, however, they lie 

 much further forward. Their area of innervation corresponds with 

 that of the combined parietal and visceral ganglia of the Gastropoda, 

 for these visceroparietal ganglia supply with nerves the two ctenidia, 

 the two osphradia, the posterior portion of the mantle, the posterior 

 adductor, and the viscera. 



The buccal or stomodseal nervous system is much reduced ; this reduction is 

 connected with the absence of a muscular pharynx and of all buccal armature. The 



