THE CAHNIVOKOUS SI-lMiS OF SOU'l'H AFIMCA. lol 



The upward growth of the skin on the right side would tend 

 to cover the respiratory opening, which would be in its usual 

 position in the right margin of the mantle. To obviate this, 

 the mantle edge, with the opening, would have to move up 

 towards the middle line, and the growth of the riglit side of 

 the shell would be retarded in consequence. And at the same 

 time the fold of the skin would be likely to grow more 

 slowly immediately opposite the opening than before and 

 behind it, thus forming a notch in the edge of the fold. 

 Eventually this fold of the skin would unite with the other 

 in the middle line excepting at the notch, which would be 

 converted into an opening above the original respiratory 

 opening in the mantle-edge. 



This hypothesis may not be correct, but it has at least the 

 merit of explaining not only the absence of an external 

 mantle in Apera, but also the dorsal and slightly asym- 

 metrical position of the respiratory opening, the fact that the 

 opening has an inner and an outer lip, the former with an 

 oblique cleft, and the peculiar sinus in the right side of the 

 shell. So far as I am aware the morphology of the mantle 

 and shell of Apera differs from that of all other Pulmonates. 



THE PEDAL GLAND. 



The pedal or supra-pedal gland attains a greater develop- 

 ment in Apera than in any other genus of snails or slugs 

 known to science. It consists essentially of a long duct with 

 glandular walls, which opens in front between the mouth and 

 the anterior end of the foot, and ends behind in a small vesicle. 



The Position and Length of the Gland. — In the herbivorous 

 genera of snails and slugs the pedal gland is usually more or 

 less embedded in the muscular tissue of the foot, and this is 

 also the case in D audebardia,^ Plutonia,- and the Tri- 

 gonochlamin^e^; but in Apera it lies freely in the lower 



' Plate, L. H., ' Zool. Jahrb.,' 1891. vol. iv, p. 525. 

 - Simroth, H., ' Nova Acta Acad. Caesar Leop. -Carol. Germ. Nat. Ciir.." 

 1891, vol. Ivi, p. 229. 



'■' Simroth, H., • Festschrift Leuckarts,' 1892, pp. 53, 55 ; etc. 



