THE CARNIVOROUS SLUGS OF SOUTH AFRICA. 17o 



these muscles were distributed ovei* a considerable area, so 

 that the strain produced by the contraction of the muscles 

 was not concentrated on a small patch of the body-wall. 

 But although it is not difficult to derive the many radially 

 disposed retractors of A. gibbon si and A. parva from the 

 single retractor of the other foi-ms, I would emphasize the 

 fact that the difference is a very great one, and not likely to 

 have been completed within a short space of time. So far as 

 I am aware the only other form which has developed radial 

 buccal retractors at all similar to those of Apera is 

 S e 1 e n o c hi a my s .^ 



Since the buccal retractor is inserted in the hind end of 

 the odontophore, it might have been expected that it 

 would receive nerves from the buccal ganglia, which inner- 

 vate the odontophore and the posterior part of the buccal 

 mass. Yet this is not the case ; for while the posterior end 

 of the reti'actor receives nerves emanating from the right 

 pedal ganglion, the front end is innervated by a pair 

 of nerves from the cerebral ganglia in all the forms 

 with a single retractor. This apparently anomalous inner- 

 vation may be explained by assuming that the buccal 

 retractor belongs to the same series of muscles as the 

 tentacular retractors, with which it is united posteriorly 

 in the great majority of snails ; and that it was originally 

 inserted, like the tentacular muscles, far forward, and in the 

 region innervated by the cerebral ganglia. Later the 

 anterior part, which consists of a right and left division, 

 would become applied to the sides of the odontophore and 

 fused with them, so that the retractor would appear to be 

 inserted in the hind end of the odontophore. This theory is 

 supported by the facts (1) that the nerves to the buccal 

 retractor and to the retractors of the lower tentacles arise 

 from the cerebral ganglia by the same roots, and (2) that 

 the longitudinal muscle-fibres forming the outer layer of the 

 sheath of the odontophore are directly continuous on each 

 side with those of the buccal retractor behind (PI. XVIII, 

 1 Simroth, H., ' Festschrift Leuckarts,' 1892, p. 55. 



