1 ORIGIN OF THE LAND AND FRESH-WATER MOLLUSCA 1 ) 



are conveniently divided into Stylomviato2:)hora} 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, Planorlis, Physa, 

 and all the Auriculidae). 

 Thus we have 



, . C Polyplacophora 



Amvhineura i a i i 



■^ (^ Aplacophora 



,, , , . J rDiotocardia 



■, J I^Monotocardia (mcl. Heteropoda) 



^ "\ ^ • ,, 7 , • , TTectibranchiata (incl. Pteropoda) 

 1 Ovist liobrancliiafa < -^t ^^^ ^ ■ , ^ 



I -* \^JSudibrancniaia - 



I ,, , , rStylommatopliora 



\ I ulmonata i -r. i. i 



\ I^Easominatophora 



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 moUuscan 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 

 Ibr 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 

 Ijeen 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 deriva- 

 tion is obscured or at best only conjectural. 



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



' (ttCXos, i)illai' ; dfj-ixara, C3'es. 

 - The Ascorjlossa are dealt with below (cliap. xv.). 



