vir MOLLUSCA—THE NERVOUS SYSTEM 137 



Although chiastoneury may be satisfactorily explained hy this 

 theory of displacement, the cause of the displacement itself has still to 

 be sought (rf. § xiv. p. 149). 



Special Remarks on the Nervous System of the Gastropoda. 



I. Prosobranchia. ('/) Diotocardia. — These are the nn)st }iiiniitive Gastropoda. 

 The ganglia are not yet very distinct, thns recalling the Amphineiim. The cerebral 

 ganglia are connected by two long commissures, the cerebral commissure running 

 forward over the pharynx, and tlie labial commissure running under the ctsopliagus. 

 The indistinctly separated buccal ganglia together form a horseshoe-shaped figure, 

 and are united on each side by a connective with the thickened root of the labial 

 commissure. 



The pleural ganglia lie close to the pedal ganglia, so that no distinct pleuro- 

 l)edal connectives can be distinguished. The pedal commissure is very short, and 

 contains ganglion cells. From each pedal ganglion, a long pedal cord runs back into 

 the foot ; these two pedal cords contain ganglion cells along their whole length, and 

 are connected by transver.se connnissures. These cords and commis.sures thus exhibit 

 the same arrangement as in the AmpMncura. The pedal cords innervate the nuis- 

 culatiu-e of the foot and the epipodium. There is only one indistinct visceral 

 ganglion, which is joined to the pleural ganglia by two pleurovisceral connectives, 

 crossed in the typical way. 



In Fissurclla only does a ganglion occur on the supraintestinal pleurovisceral 

 connective. In no other Diotocardiaii is there a ganglion at the point of dejmrture 

 of the strong branchial nerve from the pleurovisceral connective ; this nerve, how- 

 ever, forms the branchial ganglion just below the osphradium at the base of the 

 gill. Where a ctenidium, or merely an os}ihradium, is found on each side, there is a 

 branchial ganglion close to it ; where only the left (ur) gill is retained {Turhiaidcc, 

 Trochida), only the left branchial ganglion is found. Since, as a rule, the parietal 

 ganglia are wanting in the Diotocardia, and the branchial ganglia in the Monotocardia, 

 the branchial ganglia of the Diotocardia have been considered, with much prob- 

 ability, as intestinal ganglia, which have shifted away from the pleurovisceral connec- 

 tives and towards the bases of the gills. As, however, Fissurella possesses both a 

 supraintestinal and a left branchial ganglion, it would be necessary to assume that 

 an originally single ganglion had here become divided into two. 



The symmetrical pallial nerve is always connected by a pallial anastomosis 

 with the asymmetrical pallial nerves on the same side of the body. The symmetri- 

 cal pallial nerve rises out of the pleural ganglion, the asymmetrical nerves out of 

 the jiarietal ganglion, or the pleuro})arietal connective. 



The nervous .system of the Neritido-- and Helicinidce are peculiar, in that the sujira- 

 intestinal jileurovisceral connective and its corresponding ganglion are wanting. 



Docoglossa. — The only essential difference between the nervous system of Patella 

 (Fig. 117) and the typical system of other Diotocardia lies in the fact that the 

 pleural and pedal ganglia are joined by a distinct pleuropedal connective. 



{b) Monotocardia (Fig. 118). — The parietal ganglia are always present. The 

 cerebral commissure is short, and lies behind the pharynx. The labial commissiu'e 

 is wanting (except in the Falitdinidce and Ampullar idtc). The pedal cords and 

 transverse commissures are wanting (except in the Architicnioglossa : Paludinidce, 

 (Jyclophoridw, Cypraidu:). The number of visceral ganglia varies from one to 

 tiiree. 



The progressive development of so-called Zygoneury is noteworthy. In the 

 Diotocardia, a pallial anastomosis exists between the symmetrical and asymmetrical 



