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 subcsophageal ganglia. Remove the pharynx from the 
body. By slightly scraping the subcesophageal 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 subcsophageal ganglia, and 
determine so far as possible to what organs they go. 
Hxercise 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. 
