IV.] THE COMMON SNAIL. 299 



b. The anterior aorta. Its cut end is clearly visible 

 at this stage. (Cf. Sect. H. 3 b.} It perforates 

 the nerve-collar ventrally, running beneath the 

 parieto- splanchnic and above the pedal ganglia. 



c. The tentacular nen>es; arising, together with the 

 labial nerves, from the sides of the supra-ceso- 

 phageal ganglia. The nerve to the optic tentacle 

 is connected with a special ganglionic lobe. 



J. The Sense-Organs. 



i. Obtain a specimen in which the tentacles are fully 

 extended. Remove the cephalic integument and 

 tentacles, together with the nervous system, en masse; 

 pin down very carefully under water and dissect 

 under a lens. Open up the tentacles and note 



a. The retractor muscles; two ribbon-shaped bands 

 arising from the extreme anterior ends of the 

 tentacles ; they pass back to be inserted into the 

 shell axis, side by side with the larger retractors 

 already described (Sect. E. i a). 



The muscles of the smaller pair of tentacles re- 

 ceive slips from the adjacent cephalic integument. 



b. The labial tentacle. Note that the upper end of its 

 retractor muscle is darkly pigmented. Trace up 

 the tentacular nerve which accompanies the muscle; 

 it ends in a large pear-shaped ganglion, from which 

 fibres are distributed to the integument investing 

 the free end of the tentacle. 



The nerve sends a branch to the adjacent cephalic 

 integument. 



c. The ocular tentacle and eye. Its retractor muscle 

 and nerve repeat the conditions described for b ; a 



