158 HUGH WATSON. 



of the animal. In A. pure el li it is about a ninth, while in 

 A. burnupi and A. sexangula it is only about a twelfth 

 of the length of the slug. 



The diversity in the size of the teeth is even greater. In 

 Apera burnupi and A. sexangula the animal is about 

 550 times the length of the largest tooth in its radula. In A. 

 dimidia and A. pur colli the proportion is 335 : 1. In a 

 typical example of A. gibbonsi it is about 115 : 1 ; while in 

 A. parva and A. gibbonsi lupata the animal is only about 

 66 times as long as its largest tooth, which attains a length of 

 more than 0*6 mm. in the latter form. 



But the diversity found in the fadula of Apera extends to 

 other features besides its dimensions. Even in the closely 

 related forms which I am provisionally regarding as subspecies 

 of A. gibbonsi, there is a remarkable variation in the form 

 and number of the teeth (PI. XIX, figs. 123, 124, PI. XX, 

 125, 126, and text-fig. 3, a-e). The typical form has the 

 most primitive type of radula, and the others show a pro- 

 gressive adaptation to vermivorous habits. A slug that feeds 

 on worms — and it is known that this species does so — uses its 

 radula, not for rasping off pieces from its victim, but for 

 catching hold of it and drawing it back through the mouth 

 into the crop. For this purpose the teeth must be large, with 

 long sharp points for piercing the worm's skin and firm cuticle; 

 and secondly, the teeth must be shaped so as to retain their 

 position in the skin of the worm when they are pulling it back 

 into the mouth. Accordingly we find, first, a progressive 

 increase in the size of the larger teeth compared with the size 

 of the animal, and especially in the length of their cusps. 

 And in order to make room for these the number of rows is 

 diminished, and the smaller teeth towards the outer edges of 

 the radula, as well as those down the centre, become still less 

 and eventually disappear (cf . PI. XX, fig. 125, and PI. XIX, 

 fig. 123, and see also the numbers of the rows, and of the 

 teeth in each row, given on pp. 195-201). Again, we find that 

 the attachment of the teeth to the basal membrane of the radula 

 also increases in length and forms a projecting apophysis, thus 



