THE GASTROPODA 



majority of the Pectinibranchia, including the Heteropoda, and in 

 various Bullomorpha (e.g. Actaeon, 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 Rhipidoglossa. 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 



. - i - ,, ... Pleurotomaria, nervous 



are respectively given to the two moieties system, dorsal aspect, br.g, 



and tn thp o-anrrKa bnrnp nn rViPTn I'FiV 1 2^\ branchial ganglion ; c.c, cerebral 

 D ine ganglia C n^rig. 1^20;. commissure; c.p.c, cerebro- 



This disposition of the visceral com- P ed ' commissure; c.pi.c, 



_. cerebro - pleural commissure ; 



miSSUre IS common to all the JbtreptOlieura i.i, infra-intestinal portion of 



/ , i / , i \ the visceral commissure ; ta.c 



(as the name Of the group Signifies), in- labial commissure ; ot, otocyst ; 



Fio. 94. 



eluding the Heteronoda and all the forms 



*- WJ\.\JL. , /"-' , picuiiti rcjiLM' , 



formerly called " Orthoneura," i.e. forms in pi-p.c, pieuro- pedal connec- 



,., . ,, 11- 1,1 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 

 seen in the more archaic Euthyneura (which, 

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

 in various Bullomorpha (Actaeon, Fig. 57, Scaphander, Bulla, 

 etc.), 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 



