VII MOLLUSCA—THE NERVOUS SYSTEM 137 



Although chiastoneury may be satisfactorily explained by this 

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

 be sought {cf. § xiv. p. 149). 



Special Remarks on the Nervous System of the Gastropoda. 



1. Frosobranchia. (a) Diotocardia. — These are the most primitive Gastropoda. 

 The ganglia are not yet very distinct, thus recalling the Amphineura. The cerebral 

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

 fonvard over the pharynx, and the labial commissure running under the oesophagus. 

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

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

 commissure. 



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

 pedal 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 transverse commissures. These cords and commissures thus exhibit 

 the same arrangement as in the Amphineura. The pedal cords innervate the mus- 

 culature of the foot and the epipodinm. There is only one indistinct visceral 

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

 crossed in the typical way. 



In Fissiirella only does a ganglion occur on the supraintestinal pleurovisceral 

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

 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 tlie 

 gill. Where a ctenidium, or merely an osphradium, is found on each side, there is a 

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

 Trochidce), 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 Mouotocardia, 

 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, Fissiirella possesses both a 

 supraintestinal and a left branchial ganglion, it would be necessar}' 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 parietal ganglion, or the pleuroparietal connective. 



The nervous system of the Neritidc and Hclicinidce are peculiar, in that the supra- 

 intestinal pleurovisceral connective and its corresponding ganglion are wanting. 



Docoglossa. — The only essential diilerence 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. 



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

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

 is wanting (except in the Paludinidce and Amp^Ularidce). The pedal cords and 

 transverse connnissures are wanting (except in the Architccnioglossa : Paludinidce, 

 Cyclophorida;, Cyprccida). The number of visceral ganglia varies from one to 

 three. 



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

 Diotocardia, a pallial anastomosis exists between the symmetrical and asymmetrical 



