68 



They are somewhat irregular in shape and are 

 connected with each other by a commissure which passes 

 over the alimentary canal. Owing, however, to the 

 folding' of the gut and the almost vertical position of the 

 oesophagus at this point, the cerebral commissure lies 

 rather posteriorly to the gut. Both the oesophagus and 

 branches of the anterior aorta are encircled by the collar 

 formed by the cerebral ganglia, cerebral commissure, and 

 pedal ganglia. 



It will be noticed that the cerebral ganglia in the 

 whelk are by no means the most anteriorly placed. They 

 lie some little distance behind the pedal ganglia and 

 almost directly above the pleural centres. This position, 

 which may give rise to a little confusion at first, is due to 

 the course taken by the oesophagus between the proboscis 

 and the gland of Leiblein. The cerebral centres are 

 joined by very short connectives to the pedal and pleural 

 ganglia, and at this point it may be observed that there is 

 a slight asymmetry in the relative position of the right 

 and left ganglia. Two other small ganglia, the buccal 

 (fig. 42, bucc.) are in close contact with the cerebral and 

 are themselves connected by a long and delicate 

 commissure passing in front of the alimentary canal. 



Three nerves innervating the sense organs leave the 

 external face of each cerebral ganglion. The largest of 

 these is the tentacular nerve (fig. 42, tent.). It can be 

 followed quite easily in the tentacles, and bends at right 

 angles when above the ganglia to reach them. A few 

 twigs are sent to the head region from this nerve before 

 it finally penetrates the tentacle. The other two nerves 

 pass directly, the one to the eye, and the other to the 

 otocyst (fig. 42, op. n., and n. ot.). The two otocysts are 

 situated immediately below the anterior ends of the pedal 

 ganglia and just imbedded in the muscular floor. They 



