As a rule, among the Mollusca, the shell forms a passive mode of resistance to the attacks of enemies. Bivalves are enabled, by closing their valves, to baffle the assault of their smaller foes, and the operculum of univalves, both marine and land, serves a similar purpose. Many land Mollusca, especially _Helix_ and _Pupa_, as well as a number of _Auriculidae_, have the inside of the aperture beset with teeth, which are sometimes so numerous and so large that it is puzzling to understand how the animal can ever come out of its shell, or, having come out, can ever draw itself back again. Several striking cases of these toothed apertures are given in Fig. 24. Whatever may be the origin of these teeth, there can be little doubt that their extreme development must have a protective result in opposing a barrier to the entrance, predatory or simply inquisitive, of beetles and other insects. Sometimes, it will be noticed (_G_), the aperture itself is fairly simple, but a formidable array of obstacles is encountered a little way in. It is possible that the froth emitted by many land snails has a similar effect in involving an irritating intruder in a mass of sticky slime. The mucus of slugs and snails, on the other hand, is more probably, besides its use in facilitating locomotion, a contrivance for checking evaporation, by surrounding the exposed parts of their bodies with a viscid medium. [Illustration: FIG. 24.--Illustrating the elaborate arrangement of teeth in the aperture of some land Pulmonata. =A.= _Helix_ (_Labyrinthus_) _bifurcata_ Desh., Equador. =B.= H. (_Pleurodonta_) _picturata_ Ad., Jamaica. =C.= _H._ (_Dentellaria_) _nux denticulata_ Chem., Demerara. =D.= _Anostoma carinatum_ Pfr., Brazil; a, tube communicating with interior of shell. =E.= _H._ (_Stenotrema_) _stenotrema_ Fér., Tennessee, × 3/2. =F.= _H._ (_Polygyra_) _auriculata_ Say, Florida, × 3/2. =G.= _H._ (_Plectopylis_) _refuga_ Gld., Tenasserim (a and b × 2).] Some species of _Lima_ shelter themselves in a nest constructed of all kinds of marine refuse, held together by byssiferous threads. _Modiola adriatica_, _M. barbata_, and sometimes _M. modiolus_ conceal themselves in a similar way. _Gastrochaena_ frequently encloses itself in a sort of half cocoon of cement-like material. The singular genus _Xenophora_ protects itself from observation by gluing stones, shells, and various _débris_ to the upper side of its whorls (Fig. 25). Sometimes the selection is made with remarkable care; the _Challenger_, for instance, obtained a specimen which had decorated its body whorl exclusively with long and pointed shells (Fig. 26). [Illustration: FIG. 25.--_Xenophora_ (_Phorus_) _conchyliophora_ Born., concealed by the stones which it glues to the upper surface of its shell. (From a British Museum specimen.)]