288 MANUAL OF ZOOLOGY sect. 



borne on the foot. In some terrestrial forms in which an 

 operculum is absent, the opening may be closed up during 

 winter by a layer of hardened mucous matter to which the 

 name of epiphragm is applied. Lateral folds of the mantle 

 are in some of the Gastropoda (Fig. 176) reflected over 

 the shell, and may completely cover it. In some cases 

 these folds unite by their edge, so that the shell comes to 

 be enclosed in a complete sac of the mantle; such enclosed 

 shells are always imperfectly developed and incapable of 

 covering the body. Thus in Aplysia (the "Sea-hare ") and 



Fig. 174. — Shell of Solarium perspectivum from the under side. 

 (From the Cambridge Natural History.') 



other allied forms the shell is greatly reduced, thin and 

 horny, and concealed within the mantle, while in the 

 nudibranch (Fig. 177), members of the same sub-order, it 

 is entirely absent. The shell is also completely absent in 

 some of the pelagic forms {Heteropoda and Pteropodd) ; in 

 others, though present and external, it is too small to 

 enclose the animal. In the slugs the shell is vestigial and 

 concealed by the mantle. 



The Gastropoda have a well-marked head, separated 

 from the body by a constriction or neck. The mouth, 



