THE GASTROPODA 115 
the majority of the Nudibranchs (Fig. 159), and is pushed to an 
extreme in Zethys. In this case the pedal and visceral commissures 
are destitute of ganglia or nearly so on the ventral part of their 
course. It is only in the thecosomatous ‘ Pteropods” that the 
concentration of the ganglia takes place ventrad of the oesophagus, 
the dorsal part of the nervous system being formed only by a long 
cerebral commissure (Fig. 60, 1.8). 
In all Gastropoda there is an infra-oesophageal stomato-gastric 
commissure. In the more primitive forms (Aspidobranchia, Fig. 
94) the stomatogastric system originates from a sub-oesophageal 
labial commissure, but in other Gastropods from the cerebral 
ganglia. Normally this stomato-gastric commissure bears a pair of 
e 
Hie. 97. 
Nervous system of Limnaea stagnalis (dorsal aspect), as a type of the short-looped Euthy- 
neurous condition. The short visceral loop, with its three ganglia, is lightly shaded. ab, 
abdominal ganglion (+infra-intestinal); ce, cerebral ganglion; 0, osphradium; pe, pedal 
ganglion ; pl, pleural ganglion ; sp, parietal ganglion or left visceral ganglion ; opposite to it 
’ 
is the visceral ganglion of the right side, or supra-intestinal ganglion, which gives off the long 
nerve to the osphradium (in Planorbis, the osphradium and supra-intestinal ganglion are on 
the left side). (From Lankester, after Spengel.) 
ganglia situated below the radular caecum at the point of origin of 
the oesophagus from the buccal bulb (Fig. 146, g.bu). 
As regards the innervation of the different organs, the cerebral 
centres supply the head and buccal lips, the tentacles and other 
cephalic appendages, the eyes and the otocysts. The pedal ganglia 
send out nerves to the whole mass of the foot, including the epi- 
podium, and to a portion of the cervical region. The mantle and 
the organs connected with it are primitively—as in the Streptoneura 
—almost entirely innervated from the pleural centres, but they are 
also partially innervated by nerves which issue from the visceral 
commissure and the supra- and infra-intestinal ganglia, and pass to 
the ctenidia and osphradia. The last-named nerve-centres take a 
preponderant share in the innervation of the mantle in Euthyneura, 
