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 pecuHar, namely, 

 the retractor-muscle of the foot, which has its origin, in 

 common with the retractors of the tentaculse 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, Bidinms, Pujm, and Succinea oflFers nothing 

 peculiar from that of Limax. In Cflandina a third pair 

 of tentacular appendages exists. These are non-retractile, 

 auriculate in form, and originate just postero-inferiorly to 

 the base of the inferipr, retractile tentaculce, 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 



