Some recent authors, on grounds of general organisation, regard the Limnaeidae and their allies as Opisthobranchs adapted to an aerial life. It is held[24] that the Nudibranchiate Opisthobranchs have given birth to the Pulmonata Stylommatophora or land _snails_, and the Tectibranchiate Opisthobranchs to the Pulmonata Basommatophora or fresh-water _snails_. Such a view seems at first sight open to some objection from other views than those which deal simply with anatomy. The _Opisthobranchiata_ are not, to any marked extent, littoral genera, nor do they specially haunt the mouths of rivers. On the contrary, they inhabit, as a rule, only the very lowest part of the littoral zone, and are seldom found, except where the water is purely salt. In other cases, when the derivation of land or fresh-water genera is fairly well established, intermediate forms persist, which indicate, with more or less clearness, the lines along which modification has proceeded. It has, however, recently been shown that _Siphonaria_[25] and _Gadinia_,[26] which have, as has been already mentioned, hitherto been classified as _Pulmonata_, are in reality modified forms of Opisthobranchiata, which are in process of adaptation to a life partly marine, partly on land. They may therefore be regarded as supplying the link, hitherto missing, between the land _Pulmonata_ and the marine groups from one or other of which the latter must have been derived. The general consensus of recent opinion inclines towards accepting these views, some writers[27] being content to regard the _Pulmonata_, as a whole, as derived from the Tectibranchiate Opisthobranchs, while others[28] go further and regard the Stylommatophora as derived directly from the Basommatophora. =Origin of the Land Fauna= =Gasteropoda.=--(1) _Operculate._ On _a priori_ grounds, one might predict a double origin for land operculates. Marine species might be imagined to accustom themselves to a terrestrial existence, after a period, more or less prolonged, of littoral probation. Or again, fresh-water species, themselves ultimately derived from the sea, might submit to a similar transformation, after a preliminary or intermediate stage of life on mudbanks, wet swamps, branches overhanging the water, etc. Two great families in this group, and two only, seem to have undergone these transformations, the Littorinidae and the Neritidae. The derivation of almost all existing land operculates may be referred to one or other of these groups. [Illustration: FIG. 16.--Two rows of the radula of _Littorina littorea L._, × 72.]