138 



COMPAEATIVE ANATOMY 



jiallial nerves on each side. If this anastomosis were to shift ah^ng the two pallial 

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

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

 the same side of the liody. There would thus arise a new accessory ])leurointestinal 



connective, which would be symmetrical 

 and not twisted, and thus unlike the asym- 

 metrical twisted connective already existing. 

 Zygoncury 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 Rostrifera, 

 Viz. some of tlie Ccrithiidce, AmpuUariida', 

 Tu ritcllklce, Xcnophoridic, StrutMoIariidw, 

 Chenopidcc, Strombidce, Calyptrmda, and in 

 all Prohoscidifera siphunostomata and all 

 Sknoglossa). Less frequently, zygoneury 

 takes place on the left side {Ampullariidcc, 

 a few GrepiduUda, Naticida', Lamellariida: 

 Oyprceidcc). 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 lie emphasised. The con- 

 nectives uniting the various ganglia con- 

 tinuallj- shorten, so that at last anteriorly 

 on the cesojihagus there is a collection of 

 ganglia ; these are the cerebral, jileural. 

 l)edal, 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 

 overthehead(Fig. 98), a propedal ganglion be- 

 PiG. 117. —Nervous System of Patella comes differentiated from the pedal ganglion, 

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



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

 commissure ; 3, labial ganglion ; 4, buccal gau- ^^ ^,^^^^^^ ^^^^^ ^^ certainly have crossed 

 glion ; 5, cerebropleural connective ; b, cerebro- ^ i ^ r -n 



pedal connective; 7, nervus acusticus ; 8, visceral connectives, and are therefore Proso- 

 auditory vesicle ; 9, pleural ganglion ; 10, pedal branchia, and, as the rest of their organisa- 

 commissure ; 11, right, 12, left ospliradium ; tion shows, Monotocardia. The cerebral 

 13, visceral ganglion; 14, supraintestinal gan- ga„glia and the pedal ganglia (pleuropedal 



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

 pedal connectives are very long.^ 

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

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



1 Of. Pelseueer's Introduction d I'Uude des Mollnsques, 8vo, Bruxelles, 1894, pp. 

 104, 105. 



glion; 15, pedal cords; 10, indication of an 

 infraintestinal ganglion. 



