THE GASTROPODA 



the majority of the Nudibranchs (Fig. 159), and is pushed to an 

 extreme in Tethys. 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, n.s). 



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 



t? 



FIG. 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, aft, 

 abdominal ganglion (+infra-intestinal) ; ce, cerebral ganglion ; o, osphradium ; pe, pedal 

 ganglion ; pi, 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 buccaj, 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, 



