THE CARNIVOROUS SLUGS OF SOUTH AFRICA. 219 



same branch as A. dimidia, but having become still more 

 highly specialised in many ways. On the other hand, we find 

 that these species seem to be more primitive than either A. 

 dimidia or A. purcelli not only in the uniformity of the 

 teeth of the radula and the presence of a well-developed 

 central tooth in typical examples of A. gibbonsi, but also 

 in the arrangement of the intestine and liver. Now the 

 nature of these characters renders it improbable that if they 

 had once become so much modified they would revert to their 

 original condition ; and I am therefore inclined to regard the 

 resemblances between A. gibbon si and A. parva on the 

 one hand, and A. dimidia on the other, as due to parallel 

 evolution. 



Apera gibbonsi and A. parva are probably even more 

 closely related to each other than are A. burnupi and A. 

 sexangula. But A. parva is decidedly more primitive 

 than A. gibbonsi in its nervous system and long penial 

 retractor, and in possessing a true stomach. On the other 

 hand, in a few features, such as the buccal retractors and 

 hermaphrodite gland, we must regard A. parva as being 

 moi-e highly specialised than A. gibbonsi or any other 

 member of the genus. 



If these views be correct, the phylogeny of the species of 



Apera might possibly be somewhat as shown in the above 

 genealogical tree. 



VOL. 3, PART 2. 16 



