THE CARNIVOEOUS SLUGS OF SOUTH AFRICA. 229 



entirely disappeared. Thirdly, the main cusps of the teeth 

 have become far longer and more sharply pointed, so as to 

 penetrate the victim's skin, and the secondary cusps have 

 completely gone, excepting in some of the more primi- 

 tive genera of the Oleacinidse. And as all the teeth have 

 become simplified in this manner, the differentiation between 

 laterals and marginals has been lost, though it has been re- 

 acquired in most of the species of Apera and Natal in a, and 

 to some extent in Guestieria. Fourthly, the bases have also 

 become lengthened in order to prevent the teeth from being 

 bent outwards. Fifthly, the rows of teeth have become 

 sharply angled in the centre, owing to the way in which the 

 radula is forcibly drawn back over the anterior edge of the 

 odontophoral support into the narrow I'adula-sac. 



The muscles of the odontophore have become correspond- 

 ingly enlarged. In the more primitive forms the hind end of 

 the radula-sac still projects from between these muscles ; but 

 in those that are more completely adapted to a carnivorous 

 existence the odontophoral muscles have entirely surrounded 

 the radula-sac, and form a very large cylindrical structure. 



This growth of the odontophore leaves little room for the 

 development of other bulky organs in the anterior part of the 

 body, especially as it is an advantage to a carnivorous form 

 not to have a very large head and neck. I therefore suggest 

 that this is the reason why the genital ducts of carnivorous 

 snails and slugs are without large accessory organs. There 

 Avould be no room for the development of a dart-sac, for 

 example, in a snail with a very large odontophore. 



The growth of the odontophore would also tend to increase 

 the size of the body -whorl of the shell. If this were laterally 

 flattened as in the Oleacinidae, it is evident that when the 

 snail withdrew into its shell, the hind end of the odontophoi'e 

 would come to press against the outer side of the shell in the 

 neighbourhood of the posterior end of the mantle-cavity. 

 The result of this would be that the posterior limit of the 

 cavity would be pushed forwards in the middle, and with it 

 the heart and the lower end of the kidney. The upper end 



