144 HUGH WATSON. 



found in Apera itself, we must inquire whether it can 

 be discovered in the snails from which the genus may have 

 sprung. 



There are certain muscles in the odontophore of Apera 

 and Natalina — the flexor muscles of the odontophoral sup- 

 port — which, when they are contracted, tend to curve up 

 its front end. When this happens the odontophore is likely 

 to turn on to its right side, for there is not room for it 

 to become curved in a vertical plane above the pedal 

 gland, and the reproductive organs will prevent it turning 

 on to its left side and curving outwards to the right. And 

 in the only specimen of Apera that I have seen in which 

 these muscles were contracted, the odontophore was on its 

 rig'ht side, curving outwards to the left. Now in carnivorous 

 snails with a dextral heliciform or depressed shell, an 

 odontophore which curves outwards to the left will lit into 

 the body-whorl of the shell much better than one that is 

 straight, when the animal retii^es into its shell ; and the 

 oblique pull of the retractor muscles arising from the 

 columella of the shell will tend to maintain this curvature. 

 Thus one might expect to find that in such snails the 

 anterior part of the odontophore would be normally lying on 

 its right side, with the opening of the oesophagus lateral 

 instead of dorsal ; and this is exactly what has been found 

 to be the case in Paryphanta hochstetteri [Pfr.)^ and 

 Natalina trimeni {M. & P.).~ The nerve-collar Avould not 

 be rotated to any extent, as both the cerebral and ventral 

 ganglia would be held in position by the nerves which radiate 

 from them to the skin, tentacles, etc., on each side ; but 

 owing to the odontophore curving outwards to the left, the 

 cerebro-pedal and cerebro-pleural connectives would become 

 much more lengthened on the left side than on the right 

 (see Beutler's fig. 60). The buccal ganglia, however, would 

 obviously be involved in the rotation. But owing to the fact 

 that the cerebral ganglia are broader than the buccal ganglia, 



' Beutler, B.. ' Zool. Jahrb.,' 1901. vol. xiv. p. 377. pi. xxix, fig. 60. 

 ■■' Pace, S., ' Proc. Mai. Soc.,' 1895, vol. i, p. 233. 



