THE GASTROPODA 



U'n-t 



majority of the Pectinibranchia, including the Heteropoda, and in 

 various Bullomorpha (e.g. Actaean, Fig. 57) and the thecosomatous 

 " Pteropods " among the Opisthobranchia. At the same time the 

 pedal ganglia are concentrated anteriorly to form more or less 

 globular masses (Fig. 123, C, pe.g). 



Primitively the visceral commissure is somewhat extensive, and 

 its ganglionic centres are tolerably far removed from one another, 

 as may be seen in all the Streptoneura and 

 the less specialised Euthyneura (Figs. 94, 

 57, etc.). These ganglionic centres are 

 normally three in number : one is median, 

 and is called the abdominal or the visceral 

 ganglion proper (Fig. 93, XI) ; two are 

 lateral, placed right and left on correspond- 

 ing sides of the visceral commissure. The 

 ganglion on the morphologically left side 

 may be but slightly developed or may not 

 be differentiated at all, as, for example, in 

 monobranchiate Khipidoglossa. In con- 

 sequence of the torsion of the visceral mass 

 of Gastropoda, the visceral commissure is 

 normally twisted into a figure of eight ; that 

 is to say, the right moiety with the visceral 

 ganglion is situated above the alimentary 

 tract and is displaced to the left, the left 

 moiety remains below the alimentary tract, 

 but is inclined to the right (Fig. 57). Hence 

 the names supra-intestinal and infra-intestinal 



.. , ,, ... J'lenrotomaria, 



are respectively given to the two moieties system, dorsal 

 and to the ganglia borne on them (Fig. 123). 

 This disposition of the visceral com- 



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coium insure ; c.pl.<, 

 cerebro pleural commissure : 

 miSSUre IS Common to all the StreptOlieUra i.i, infra-intestinal portion of 



(as the name of the group signifies), in- 

 cluding the Heteropoda and all the forms 

 formerly called "Orthoneura," i.e. forms in pi-p-c, piem-o"- pedal 



, . , X 11 IT lit tive s -i supra-intestinal part 



Which the Visceral loop Was believed to have of the visceral commissure ; 



never been twisted ; it may also be clearly (After^M^woS^ 

 seen in the more archaic Euthyneura (which, 



as has been explained above, are detorted Gastropods), for instance, 

 in various Bullomorpha (Actaem, Fig. 57, Scaphander, JBulla, 

 CLC.), and in Chilina. But in the three last-named genera the 

 detorsion of the visceral commissure is already manifest, that is 

 to say, its supra-intestinal moiety shows a tendency to return to 

 the lower side of the alimentary tract, and its sub-intestinal moiety 

 tends to return to the left side. This detorsion of the visceral 

 commissure is complete in the rest of the Euthyneura, as may be 



8 



