THE CABNIYOU'OUS SLUGS OF SOUTH AFRICA. 235 



with Apera; indeed, if we take into consideration both the 

 internal and external characters, I do not think that any- 

 other genus resembles Apera so closely as does Testacella. 

 The pedal gland of Testacella lies free in the body-cavity 

 as it does in Apera. Both genera are opisthopnemnic, and 

 in both the primary ureter opens at the posterior end of the 

 mantle-cavity. The reproductive organs of the two genera 

 are on the whole very similar, and in both the right tentacular 

 retractor crosses the penis. But if we compare the genera 

 more closely we find that there are several small differences 

 which it is not easy to explain away. In the first place, the 

 auricle in Testacella has come to lie directly behind the 

 ventricle, whereas in Apera the heart has not been rotated 

 so far. Yet Testacella cannot be derived from Apera, 

 because it still retains an external shell. Secondly, the foot 

 of Testacella contains numerous dermal mucous glands, 

 while that of Apera has none. Thirdly, the pedal gland of 

 Testacella has no terminal vesicle. Fourthly, the left 

 parietal ganglion has not become fused with the abdominal 

 ganglion in Testacella, but all the visceral ganglia remain 

 separate. Fifthly, the arrangement of the odontophoral 

 muscles, and the structure of the odontophoral support, and 

 even the blood-supply of the odontophore, show surprising 

 differences in the two genera, as we have already seen. And, 

 lastly J there is the difference in the geographical distribution 

 of the two genera. On the other hand, the resemblances are 

 found to be largely of a negative character, if we except 

 those which might be due to the common acquisition of 

 vermivorous habits by both forms. Therefore it will be well 

 to look further before we assume that Apera is related to 

 Testacella. It is true that there are no other carnivorous 

 genera in which the shell has become reduced, but it is 

 possible that the nearest living allies of Apera may have 

 quite large shells, for when once the presence of a shell is 

 found to be a disadvantage, its degeneration probably takes 

 place rather rapidly. 



Pilsbry has suggested that Apera may possibly be allied 

 VOL. 3, PART 2. 17 



