THE CARNIVOROUS SLUGS OF SOUTH AFRICA. 225 



vorous forms and on worms, for most ai'thropods move too 

 quickly and are too well protected by their cliitinous exo- 

 skeleton to fall a prey to snails. Now worms burrow in the 

 soil, and snails hide themselves in crevices and retire deeply 

 within their shells when attacked ; it is therefore evident that 

 a carnivorous snail will find the presence of a bulky un- 

 yielding shell on its back a great inconvenience when it is 

 trying to get near its victim. In order to obviate this diffi- 

 culty the shell has become modified in various ways. In the 

 first place we find that in Paryphanta it has become more 

 or less flexible owing to the degeneration of the inner cal- 

 careous layer. Secondly, the shape of the shell has become 

 altered in many of the carnivorous genera. In Diplom- 

 phalus, for example, the shell has become greatly flattened. 

 This will enable the animal to penetrate into crevices, but 

 it is obvious that the breadth of the shell will have to be 

 reduced as well as its height, if the snail is to crawl into 

 narrow holes. Now the only way in which both the height 

 and breadth of a depressed or heliciform shell can be reduced 

 is by the curvature of the axis or columella, until its 

 direction corresponds more nearly with that in which the 

 animal moves ; and this is what has occurred in that remark- 

 able genus of carnivorous snails, Streptaxis. If, however, 

 the shell has a raised spire, the columella naturally takes up 

 a position parallel to the direction in which the animal moves, 

 and in this case it is only necessary for the height of the 

 spire to be increased in ordei- to reduce the diameter of the 

 shell. This is what has taken place in the large genus 

 Ennea, in which only the young form retains a comparatively 

 low spire. In Diaphora this principle is carried to an 

 extreme, some species having shells with greatly produced 

 spires composed of as many as twenty whorls. Indeed, so 

 long does the spire become that the animal can no longer 

 occupy the whole of it, and secretes a new internal wall 

 cutting oif part of the upper whorls, which may become 

 decollated, as in D. teles copium Mlldff. A more efficient 

 method of reducing the diameter of the shell (but one 



