500 ELEMENTARY BIOLOGY. [CHAP. 



large terminal ganglion is present, in relation to 

 the integument investing its free end. 



The visual organ is perceptible as a minute 

 black spot, lying to the outer side and a little in 

 front of the ganglion. A delicate optic nerve can 

 be traced back from it to the tentacular nerve, 

 which it enters a short distance behind the above- 

 named ganglion. 



2. Remove the eye together with a small portion of its 

 related integument, and examine in water under a 

 low power. Note — 



a. The choroid; a pigmented investment for the eye, 

 under cover of which the optic nerve passes to 

 reach the retina. 



/'. The crystalliiie lens ; seen to project freely beyond 

 a. in front. It lies under cover of a delicate 

 cornea. 



3. The auditory organ. Pin one half of the nerve-collar 

 down under water, and, having dissected off the 

 superficial stratum of its sheath, examine with a lens. 

 Note — 



a. The auditory nen'e ; a delicate undulating trunk, 

 lying in the interspace between the cerebro-pedal 

 and cerebro-splanchnic commissures. Follow it 

 up — it arises from the supra-oesophageal ganglion. 

 Below, it enters the otic vesicle^ now seen as 

 • a minute white speck, overlying the point of 

 apposition of the pedal and parieto-splanchnic 

 ganglia. 



h. The otic vesicle. Remove this, together with a 

 small portion of its surrounding tissues, and 



