174 HUGH WATSON. 



fig". 122). Tliis view would also explain how it is that in A. 

 gibbonsi the nerves which innervate the buccal retractor in 

 the other species merely unite with the sheath of the 

 odontophore towards the anterior end of that organ. For 

 we may suppose that in A. gibbonsi and A. parva the 

 odontophore has grown back further than in the other species 

 between the two halves of the buccal retractor, which have 

 thus become applied to its sides, and consequently the free 

 radial retractors of these forms are homologous with only the 

 posterior half of the long retractor found in the remaining 

 species. 



We have seen that the origin of the buccal retractor in such 

 forms as Apera dimidia is to the right of the middle line, 

 and that its posterior end is innervated exclusively by branches 

 of the right posterior pedal nerve (PI. XV, fig. 75). Now the 

 foot is a symmetrical organ, and the anterior part of the 

 retractor is certainly symmetrical, as is proved by its 

 innervation ; it might, therefore, have been expected that 

 the posterior part of the buccal retractor would have been 

 symmetrical also, unless it had shifted somewhat to the left 

 because of the pressure of the reproductive organs on the 

 right side, as in the case of Daudebardia saulcyi {Bgt.) 

 and Testacella gestroi [Issel)} Why, then, does the 

 buccal reti'actor spring from the right side of the floor of 

 the body-cavity in Apera? When an animal possesses a 

 feature which is asymmetrical for no apparent reason, the 

 explanation of that asymmetry is usually to be found by a 

 consideration of the animal's phylogeny. It is highly 

 probable that Apera has been evolved from a snail possess- 

 ing a depressed or heliciform dextral shell. Now when such 

 a shell is in its natural position upon the back of the animal, 

 its columella is to the right of the aperture. Consequently 

 the columellar muscle, from which the buccal retractor 

 springs, will be towards the right side of the animal. Then, 

 when the shell degenerates, all the upper part of the colu- 

 mellar muscle will disappear, and only the lower part, or 

 • Plate, L. H., 'Zool. Jahrb.,' 1891, vol. iv, p. 596. 



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