THE GASTROPODA 
113 
majority of the Pectinibranchia, including the Heteropoda, and in 
various Bullomorpha (¢.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.q). 
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 
Fic. 94. 
the names supra-intestinal and infra-intestinal 
are respectively given to the two moieties 
and to the ganglia borne on them (Fig. 123), 
This disposition of the visceral com- 
missure is common to all the Streptoneura 
(as the name of the group signifies), in- 
cluding the Heteropoda and all the forms 
formerly called ‘“Orthoneura,” ic. forms in 
which the visceral loop was believed to have 
never been twisted ; it may also be clearly 
seen in the more archaic Euthyneura (which, 
Pleurotomaria, nervous 
system, dorsal aspect. br.g, 
branchial ganglion ; ¢.c, cerebral 
commissure; ¢.p.c, cerebro- 
pedal commissure; e.pl.c, 
cerebro - pleural commissure ; 
i.i, infra-intestinal portion of 
the visceral commissure ; la.c 
labial cominissure ; ot, otocyst ; 
pa.n, pallial nerve; pe.c, pedal 
cord ; pl.c, pleural centre ; 
pl.p.c, pleuro-pedal connec- 
tive; s.i, supra-intestinal part 
of the visceral commissure ; 
st.g, stomato-gastrie ganglion. 
(After F. M. Woodward.) 
as has been explained above, are detorted Gastropods), for instance, 
in various Bullomorpha (Actacon, Fig. 57, Scaphander, Bulla, 
ete.), 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 
