202 INTRODUCTION. 
When the foot is protruded from the shell, every part 
of the exterior surface of the turbinated mass is still in 
contact with the interior surface of the latter, and is re- 
tained so by means of the comparatively capacious pul- 
monary chamber. When the foot is retracted, it is at 
the expense of the latter cavity ; so that the pulmonary 
chamber of the testaceous genera is as much larger than 
that of the naked genera as the size of the foot super- 
added, whilst the extent of the pulmonary net-work of 
blood-vessels remains the same. 
The testacea have a muscle which is peculiar, namely, 
the retractor-muscle of the foot, which has its origin, in 
common with the retractors of the tentacular and buccal 
body, from the columella of the shell. Narrow at its 
commencement, it increases in breadth, splits into several 
bands, and diverges as it descends to get its insertion 
into the whole of the inner margin of the excavation of 
the foot, excepting anteriorly, where its place is occupied 
by the retractor of the buccal body. 
The head occupies the anterior portion of the foot, and 
in Helix, Bulimus, Pupa, and Succinea offers nothing 
peculiar from that of Llmax. In Glandina a third pair 
of tentacular appendages exists. These are non-retractile, 
auriculate in form, and originate just postero-inferiorly to 
the base of the inferior, retractile tentacuhc, and project 
horizontally backward. 
The body of the testacea, like that of slugs, has two 
great cavities. The visceral cavity includes the greater 
part of the turbinated mass, and the excavation of the 
