THE CARNIVOROUS SLUGS OF SOUTH AFRICA. 143 



odontophore in the circular muscles of the sheath, which it 

 doubtless innervates. The inner and larger branch goes 

 more deeply. It runs down in the suspensor muscles of the 

 odontophoral support and then divides, one division running 

 forwards and the other backwards in the angle of the groove 

 between the lower ends of the suspensor muscles and the 

 origin of the lateral retractors of the radula.^ 



The nerves which innervate the central part of the odonto- 

 phore are widely separated from the other buccal nerves in 

 their origin. They arise from the inner sides of the buccal 

 ganglia, or even from the buccal commissure, and consist of 

 two rather large nerves and usually one or two others which 

 are very much smaller. These nerves pass through the sheath 

 of the odontophore, and run back with the median dorsal 

 muscle until it dips down between the lateral retractoi's of the 

 radula. They then curve forwards again at a lower level, and 

 are distributed where the retractors are inserted on the 

 radula-sac. But the most remarkable feature about these 

 nerves is their asymmetrical origin. The left of the two 

 main nerves always arises nearer the middle line than the 

 right ; indeed, it might be said that while the right nerve 

 arises from the inner side of the right ganglion, the left nerve 

 arises from the commissure, though towards its left end (see 

 especially PI. XV, fig. 71). This asymmetry seems to be a 

 constant feature of the genus Apera, for I have examined 

 microscopically the buccal ganglia of five specimens, belonging 

 to three different species, and they all show it. Yet it is 

 very surprising, for the odontophore is one of the few organs 

 which is generally supposed to have escaped the effects of 

 the torsion which has played such havoc with the symmetry 

 of G-astropods. And, in fact, the odontophoral muscles of 

 Apera do seem to be quite symmetrically arranged, and 

 the odontophore occupies an approximately symmetrical 

 position, and even the extrinsic buccal retractors show no 

 traces of asymmetry excepting towards the hind end of 

 the animal. »Since the cause of this asymmetry cannot be 

 ' See pp. 166, 167. 



