C, A Nudibranchiate Opisthobranch, Aeolis peregrina, Naples: f, foot; c, cerata.
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
| Gasteropoda | Amphineura | Polyplacophora |
| Aplacophora | ||
| Prosobranchiata | Diotocardia | |
| 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.
Origin of the Land and Fresh-water Mollusca.—The ultimate derivation of the whole of the land and fresh-water molluscan fauna must, as has already been remarked, be looked for in the sea. In certain cases the process of conversion, if it may be so termed, from a marine to a non-marine genus, is still in progress, and can be definitely observed; in others the conversion is complete, but the modification of form has been so slight, or the date of its occurrence so recent, that the connexion is unmistakable, or at least highly probable; in others again, the modification has been so great, or the date of its occurrence so remote, that the actual line of derivation is obscured or at best only conjectural.
Fig. 7.—A, the common cockle (Cardium edule L.). B, Adacna plicata Eichw., Caspian Sea. C, Didacna trigonoides Pall., Caspian Sea.