122 INVERTEBRATE ZOOLOGY 



surface a pair of nerves runs to the posterior end of the 

 pharynx, where they meet a pair of small pharyngeal ganglia. 

 The supracesophageal ganglia are connected by broad connectives 

 with the subcesophageal ganglia. Remove the pharynx from the 

 body. By slightly scraping the suboesophageal ganglia with a 

 small scalpel, it will be seen to consist of two principal gan- 

 glionic masses. The forward mass is a pair of ganglia, the 

 pedal ganglia ; the hinder mass consists of the large visceral ganglia, 

 at the side of which is the pair of small pleural ganglia. Observe 

 the nerves radiating from the suboesophageal ganglia, and 

 determine so 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 imbedded in the pedal ganglia. In 

 order to see them cut off the subcesophageal ganglion, mount 

 it in glycerine and examine it under a compound microscope. 

 The auditory nerves are very delicate and come from the supra- 

 cesophageal ganglia. 



