INTRODUCTION. 
‘ 
HE 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. 
PHYLOGENETICALLY, Testacella and Daudebardia have probably been 
derived from an identical stock, but do not stand in linear sequence. 
The Daudebardie 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 Testacell are restricted to Western and Southern Europe and North- 
western Africa, being bounded towards the east by the range of the nearly- 
allied Daudebardia. 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. 
