I02 PHYLUM MOLLUSCA 



the pair of small pleural ganglia. Observe the nerves radiating 

 from the suboesophageal ganglia, and determine as far as possible 

 to what organs they go. 



Exercise 9. Make a semidiagrammatic drawing of the nervous system. 



Organs of Special Sense. The eyes of the snail at the end of the 

 posterior tentacles have already been noted. They are easily seen 

 in a large animal which has its tentacles extended. The snail is 

 also provided with a pair of auditory organs. They consist of 

 two small sacs embedded in the pedal ganglia. In order to see 

 them cut off the suboesophageal ganglion, mount it in glycerine, 

 and examine it under a compound microscope. The auditory 

 nerves are very delicate and come from the supraoesophageal 

 ganglia. 



