THE CAKNIVOliOUS SLUGS OF SOUTH AFKIOA. 223 



snails and slugs^ a group which is usually kuown as the 

 A gnat ha, since very few of these forms possess a jaw. But 

 to state this is to say veiy little about the affinities of these 

 genera, for it is doubtful whether the various carnivorous 

 snails are at all closely related to one another, some authorities 

 asserting that the tribe A gnat ha is polyphyletic. 



Certain eminent malacologists have held that the Stylom- 

 matophora may be divided into two groups, one mainly 

 herbivorous, the other mainly carnivorous ; and they would 

 account for the diversity of the carnivorous forms by 

 supposing that they had evolved in a manner parallel to the 

 gnathophorous families, which show a similar diversity. 

 This view was well expressed by Fischer, who, writing so 

 long ago as 1873, said: "Je suis persuade qu'on pourra con- 

 stituer, dans quelques aunees, une serie d^Agnathes ou Testa- 

 cellidas ayant une grande extension, et dont les genres seront 

 representatifs de ceux des Helicida3, ainsi que, chez les Mammi- 

 feres, certains groiipes des Marsupiaux i-epresentent d'autres 

 groupes des Monodelphes/^^ On the other hand, many 

 modern authorities maintain that the various families of 

 carnivorous snails and slugs have been evolved independently 

 from different families of the Gnathophora, and that they 

 are in reality no more nearly related to one another than are 

 the European and Tasmanian wolves. The characters which 

 the agnathous families undoubtedly possess in common they 

 would explain as being due to convergence, brought about by 

 their common acquisition of carnivorous habits.- Now there 

 is no a priori objection to this second view, for we know 

 that many gnathophorous forms occasionally devour animal 

 food, and there seems to be no reason why this should not 

 have become the usual diet in some cases. The question, 

 therefore, can only be settled by a careful examination of the 

 comparative anatomy of the various families included in the 



' • Joiu-n. de Conchyl.," vol. xxi, p. 12. 



• See an iuterestiug paper by Dr. Simroth CNaturwiss.Wochensclir.,' 

 1901, vol. xvii, pp. 109-114, 121-127, 137-140), m which he has ably 

 advocated tlie polyphyletic origin of the carnivorous snails and slugs. 



