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 te7itacidar nerves ; arising, together with the 

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

 phageal gangha. 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, ejt 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. li\\Q labial tentacle. Note that the upper end of its 

 retractor muscle is darkly pigmented. Trace up 

 the tentacular iierve \^\\\q\\ accompanies the muscle; 

 it ends in a large pear-shaped ^^^/^/w;/, 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 



