134 INVERTEBRATE ZOOLOGY 
join it with the subcesophageal mass, which is composed of the 
visceral, pedal, and in front of the latter, the brachial ganglia. 
Connected with the sides of the cerebral mass are the two optic 
nerves, which widen out to form the large optic ganglia, and run- 
ning forward from it are two small nerves which connect it 
with the suprapharyngeal ganglia, a small mass above the hinder 
end of the pharynx. From these ganglia small nerves pass 
around the csophagus to the pair of subpharyngeal ganglia. 
From the forward surface of the subcesophageal mass, é.e., from 
the brachial ganglia, ten nerves pass off to the arms. These 
may be seen on the inner surface of the head after the removal 
of the pharynx. From the hinder surface of the visceral 
ganglia pleural nerves run to the stellate ganglia in the mantle. 
Trace these nerves from the stellate ganglia to their source. 
The pen. Make a longitudinal slit in the mantle on the back 
of the animal and remove the pen; it will be seen to lie quite 
loosely in its sac. 
Exercise 8. Draw the pen. 
