THE CAENIVOKOLTS SLUGS OF SOUTH AFRICA. 169 



end is abruptly Lent downwards, and the outer longitudinal 

 muscles of the sheath form a pad extending from the recurved 

 end a short distance forwards along the ventral surface. (2) 

 The lateral retractors towards the posterior end of the 

 odontophore are attached to the odontophoral support just 

 within its edges ; further forward they are attached to the 

 outer sides of the support as in Apera. (3) The median 

 dorsal muscle is extremely slender, and the terminal retractor 

 is also very weak, being almost devoid of muscular fibres. 

 (4) The circular muscles of the sheath are much thicker 

 laterally and ventrally, and the suspensor nmscles of the 

 support are also much thicker than in Apera. In all other 

 respects the structure of the odontojjhore is strikingly similar 

 to that found in Apera, and especially to that of A. 

 gibbonsi; for in Natalina quekettiana a thin layer of 

 longitudinal muscles covers the lower surface of the odonto- 

 phoral support, and some of the posterior retractors are not 

 attached to the support, but arise from the sheath of the 

 odontophore opposite to the insertion of the upper branches 

 of the extrinsic buccal retractors — features which are also 

 found in A. gibbonsi. 



The chief differences mentioned above may be easily 

 explained. The curvature of the odontophore is doubtless 

 due to the fact that Natalina possesses a large spiral shell. 

 The greater thickness of the constrictor muscles running in a 

 circular direction, both those in the sheath and those beneath 

 it, is probably connected with the circumstance that Natalina 

 feeds on snails rather than on worms ; for, as Woodward has 

 pointed out, the contraction of these muscles will press the 

 radula against the body of its prey, as it tears the teeth 

 through its victim's flesh, a thing that a vermivorous form does 

 not do. The reduction of the median dorsal muscle and the 

 terminal retractor, which might be regarded as its continua- 

 tion, may be due to the fact that Natalina rasps off portions 

 of its prey, and therefore does not require to enlarge the 

 opening of the oesophagus to such an extent as an animal 

 that swallows worms whole. We see, therefore, that the 



