INTRODUCTION. 



i^gvHE naked mollusks, familiarly termed " slugs," judged by their 

 general external aspect, are apparently a closely -related group of 

 animals, but when their organization is closely studied, they are found to 

 be not so nearly allied as they outwardly appear, as the tendency to nudity 

 is one that has affected many diverse families, being a stage of evolution 

 to which almost every group has furnished examples and to which many 

 are undoubtedly tending. 



Though it was considered convenient to devote a volume exclusively to 

 the naked species, this was not merely because they could be — from one 

 point of view — suitably placed together, as having arrived at a similar 

 phase of the shell degeneration, many testaceous forms are probably now 

 undergoing, but partly because the phylogenetic relationship of some of 

 the groups is still obscure, and we shall probably require to look for such 

 progenitors, if they be not really extinct, in some of the less advanced 

 regions of the globe. 



Phylogenetxcally, Testacelln and Daudebardia have probably been 

 derived from an identical stock, but do not stand in hnear sequence. 

 The Daiidebardiw have retained their terrestrial habits, while Testacella 

 has become more especially adapted to a subterranean existence, probably 

 thereby entering upon a course of deterioration and degradation of type. 

 Their remote testaceous progenitors have probably been long ago expelled 

 from the European region, and must be sought for in the more distant 

 and weaker regions "of the earth. 



The TestacellcB are restricted to Western and Southern Europe and North- 

 western Africa, being bounded towards the east by the range of the nearly- 

 allied Daudehardice. The group originated in the European region, but 

 some authors have erroneously surmised that the family was evolved 

 within the weak but mysterious recesses of Central Asia, afterwards 

 migrating therefrom by way of southern Europe to this country. 



