138 



COMPARATIVE ANATOMY 



CHAP. 



pallial nerves on each side. If this anastomosis were to shift along tlie 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 Mostrifera, 

 viz. some of tlie Ccrithiiikc, Anqmllariidce, 

 Tu riteHidce, Xenoplioridcc, Struthiolariidu; 

 Chenopidcc, Slrombidm, Calyptrceidcc, and in 

 all Prohosddifera siphonostomata and all 

 Stenoglossa). Less frequently, zygoneury 

 takes place on the left side (Ampullariidce, 

 a few CrepiduUda:, Naticida:, Lamellariidcc 

 Oypraddce). In other Prosohranchia there 

 is only a pallial anastomosis on each side, as 

 in the Diotocardia ; the nervous system is 

 then called dialyneurous. 



The progi-essive concentration of the 

 central nervous system of the Monotocardia, 

 which keeps pace with the development of 

 zygoneurj-, must be emphasised. The con- 

 nectives rmiting 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 

 iu 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 he- 

 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 j-equires fresh investigation. So far as we 

 commissure ; 3, labial ganglion ; 4, buccal gau- , , , j.i j. . n t 



glion ; 5, cerebropleurJcormective ; 6, cerebro- ^^ P'^'^^f * ^now, they certamly have crossed 

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

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

 commissure ; 11, right, 12, left osphradium ; tjon shows, Monotocardia. The cerebral 

 13, visceral ganglion ; 14, supraintestinal gan- „„„„!,• „„j ^-l.„ ,.^j i t / i n i 



,1,1 ,,»■,»• « ganglia ana the pedal ganglia (pleuropedal 

 15, pedal cords; 16, indication of an ,. „, . ' 6 = \F* "i"i"i>JO'i 



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 



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

 infraintestinal ganglion. 



1 Cf. Pelseneer's Introduction I'etude des MoUusqwes, 8vo, Bruxelles, 1894, pp. 

 104, 105. 



