138 



COMPARATIVE ANATOMY 



CHAP. 



pallial nerves on eacli side. If this anastomosis were to shift along the two pallial 

 nerves of one side to their places of origin, i.e. the ganglia from which they spring, 

 it would become a pallial connective uniting the pleural and parietal ganglia of 

 the same side of the body. There would thus arise a new accessory pleurointestinal 



connective, which would be 'symmetrical 

 and not twisted, and thus unlike the asym- 

 metrical twisted connective already existing. 

 Zygoneury thus depends on the development 

 of such a pallial connective. In the large 

 majority of cases in which it occurs it takes 

 place on the right side (a few fiostrifcra, 

 viz. some of the Cerithiidce, Ampullariidce, 

 Turitellidce, Xenophoridce, Struthiolar ii<l<i , 

 Chenopidce, Strombidce, Calyptrceidce, and in 

 all Probosddifera siphonostomata and all 

 Stenoglossa}. Less frequently, zygoneury 

 takes place on the left side (Ampullariidce, 

 a few Crepidulidcc, Naticidce, Lamellariidcc 

 Cyprceidcc}. In other Prosobranchia there 

 is only a pallial anastomosis on each side, as 

 in the Diotocardia ; the nervous system is 

 then called dialyneurous. 



The progressive concentration of the 

 central nervous system of the Monotocardia, 

 which keeps pace with the development of 

 zygoneury, must be emphasised. The con- 

 nectives uniting the various ganglia con- 

 tinually shorten, so that at last anteriorly 

 on the oesophagus there is a collection of 

 ganglia ; these are the cerebral, pleural, 

 pedal, infraintestinal, and supraintestinal 

 ganglia, all lying close together, to which 

 must be added the small buccal ganglia. 

 Only the visceral ganglia remain far back 

 in the visceral dome. 



In Natica, where the anterior part of the 

 foot is strongly developed, and is bent back 

 over the head (Fig. 98), a propedal ganglion be- 

 FIG. 117. Nervous System of Patella comes differentiated from the pedal ganglion. 

 (adapted from figures by Pelseneer and The nervous system of the Heteropoda 

 Bouvier). 1, Cerebral ganglion ; 2, cerebral requires fresh investigation. So far as we 



fc th certainl haye crossed 



J / . 



visceral connectives, and are therefore Proso- 



branchia, and, as the rest of their organisa- 

 tion shows, Monotocardia. The cerebral 



^ and the pedal ganglia (pleuropedal 

 & 6 

 ganglia ?) are far apart, so that the cerebro- 



pedal connectives are very long. 1 

 II. Opisthobranchia. The nervous system of this order, in which the typical 

 Gastropodan ganglia are developed, is further characterised : (1) by the absence of 



commissure ; 3, labial ganglion ; 4, buccal gan- 

 glion ; 5, cerebropleural connective ; 6, cerebro- 

 pedal connective; 7, nervus acusticus; 8, 

 auditory vesicle ; 9, pleural ganglion ; 10, pedal 

 commissure ; 11, right, 12, left osphradium ; 

 13 visceral ganglion ; 14, supraintestinal gan- 

 glion ; 15, pedal cords ; 16, indication of an 

 infraintestinal ganglion. 



1 Cf. Pelseneer's Introduction d V etude des Mollusques, 8vo, Bruxelles, 1894, pp. 

 104, 105. 



