COOKE : EVOLUTION IN THE MOI.LUSCAN RADUI.A. I47 



is increased, and the extreme marginals closely resemble those of 

 Ghmdina and Testacella. 



What happens in the case of the radula of the confirmed carnivora 

 (^Glatidina, Streptaxis, Streptostele, Gibbus, Rhytida, Daudebardia, 

 Testacella, etc.) has been well discussed by Hugh Watson in his paper 

 on "The Carnivorous Slugs of South Africa."^ "In the subspecies 

 o{ Apera gibbo/ni," he says, "the typical form has the most primitive 

 type of radula, and the others show a progressive adaptation to car- 

 nivorous habits. . . A slug that feeds on worms uses its radula . . . 

 for catching hold of its victim, 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 . , . they 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 outward edges of the radula, 

 as well as those [teeth] down the centre, become still less and 

 eventually disappear." He goes on to show how the shape of the 

 teeth becomes modified in order to prevent the worm from escaping. 

 In one subspecies the cusps are curved into hooks and slightly 

 broadened towards the points ; in another they are barbed on the 

 lower side, as in Testacella ; in a third, all the larger teeth are doubly 

 barbed, one barb being nearer the point of the tooth than the other. 



As further illustrating the process of development, it may be added 

 that the genera Daudebardia, Sele?iites, and Phitonia retain a modified 

 limacidan jaw combined with a markedly testacellidan radula. 



Among the carnivorous marine moUasca, the Toxoglossa {Conidce, 

 Te'rebridce, Cancellariidce, TurridcE, etc.), exhibit a radula which, as a 

 rule, consists of sword-shaped marginals only, median and lateral teeth 

 having been lost. To compensate for this loss, or perhaps it would 

 be more correct to say as occasioning it, the surviving marginals are 

 often very large, gathered in one or two bunches, and in Conus 

 hooked, barbed, and furnished with a poison gland and duct. The 

 ' bite ' of some species of Conus will draw blood from a man's hand. 

 Clavatula retains a small central tooth. One group alone of the 

 TurridcR {^Spirotropis) retains both median and lateral teeth, flanked 

 by the sword-shaped marginals, thus giving the clue to the true 

 explanation of what has taken place in the Toxoglossa as a Whole. 



The Rhachiglossa are distinguished by a radula in which there is 

 a central tooth, flanked by a single lateral on each side, but no marg- 

 inals (formula i.i.i). The median tooth is usually smaller than the 



I Ann. Natal Museum, III, 1915, p. 158. 



