I ORIGIN OF THE LAND AND FRESH-WATER MOLLUSCA II 



are conveniently divided into Stylommatophora} 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 

 Basommatopliora^ in which the eyes are at the base of the ten- 

 tacles, which are not retractile (^Limnaea^ Flanorbis, Phy§a^ and 

 all the Aiiriculidae^. 



Thus we have 



A 1' { PohTDlacophora 



Amphmeura \ . ^ ^ 



^ [ Aplacophora 



Prosobranchiata 



J 



J Diotocardia 



J [ Monotocardia (incl. Heteropoda) 



Gasteropoda r Tectibranchiata (incl. Pteropoda) 



[ iS udibranchiata ^ 



n 1 , { Stylommatophora 



Pulmonata \ ^-^ , , 



[ 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 MoUusca — 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 con- 

 nexion 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. 



This passage from a marine to a non-marine life — in other 



1 <TTv\o^^ pillar ; b/xixara, eyes. 



2 The Ascoglossa are dealt with below (chap. xv.). 



