[Illustration: FIG. 5.--=A=, A Tectibranchiate Opisthobranch, _Umbrella mediterranea_ Lam., Naples: _a_, anus; _br_, branchia; _f_, foot; _m_, mouth; _rh_, rhinophores; _sh_, shell. =B=, A Pteropod, _Hyalaea tridentata_ Forsk., Naples: _sh_, shell; _l_, _l_, swimming lobes of foot. =C=, A Nudibranchiate Opisthobranch, _Aeolis peregrina_, Naples: _f_, foot; _c_, cerata.] [Illustration: FIG. 6.--Examples of--=A=, Pulmonata Basommatophora, the common _Limnaea peregra_ Müll.: _e_, _e_, eyes; _t_, _t_, tentacles. =B=, Pulmonata Stylommatophora, _Helix hortensis_ Müll.: _e_, _e_, eyes; _t_, _t_, tentacles; _p. o_, pulmonary orifice (the position of the pulmonary orifice in _Limnaea_ will be seen by reference to Fig. 101).] (4) The _Pulmonata_[15] are asymmetrical air-breathing non-marine Mollusca, generally, but not always, furnished with a shell. The sexes are always united in the same individual, and the operculum is always wanting, except in _Amphibola_. They are conveniently divided into _Stylommatophora_,[16] in which the eyes are at the tip of the upper tentacles, which are retractile (_Helix_, _Limax_, _Bulimus_, and all true land slugs and snails), and _Basommatophora_, in which the eyes are at the base of the tentacles, which are not retractile (_Limnaea_, _Planorbis_, _Physa_, and all the _Auriculidae_). Thus we have { _Amphineura_ { Polyplacophora { { Aplacophora { { _Prosobranchiata_ { Diotocardia =Gasteropoda= { { Monotocardia (incl. Heteropoda) { { _Opisthobranchiata_ { Tectibranchiata (incl. Pteropoda) { { Nudibranchiata[17] { { _Pulmonata_ { Stylommatophora { { Basommatophora The relation of the four great Orders to one another will be better discussed when we come to deal with each Order separately. The problem of the origin and mutual relationship of the various forms of molluscan life is of extreme subtlety, and its solution can only be approached after a comprehensive survey of many complicated anatomical details. But there is one branch of the Mollusca--the land and fresh-water genera--whose origin is, comparatively speaking, of recent date, and whose relationships are therefore less likely to have suffered complete obliteration.