5^22 Causes and Course of Organic Evolution 



and heteropods) or fixed animal that is characteristic of the 

 above named divisions of molluscs, developments proceed 

 that can often alone be phylogenetically explained in terms 

 of a rotiferan ancestry. We shall therefore shortly consider 

 the groups in succession. 



The Gasteropoda seem in many points to continue rotiferan 

 features more directly than other moUuscan divisions. But, 

 in order to conduce to a better tracing of subsequent changes, 

 it may here be suggested as likely that from a fresh-water 

 proto-gasteropodous ancestry three lines of evolving and diverg- 

 ing specialization proceeded. First: a series that retained 

 the straight body and alimentary canal with posterior anus, 

 that retained two simple excretory tubes, four to two antennse 

 or tentacles, and that retained a dioecious state, gave rise to 

 genera that remained in fresh water as the Helicinida, Hydro- 

 biida, etc., while extensive migration into the sea originated 

 the Patellida, Pleurotomarida, etc., that early formed a heavy 

 calcareous shell. Here, owing to weight of shell, progression 

 by right- or by left-handed motion, relation of sex and ovi- 

 position, increasing unilateral muscular strains caused gradual 

 twisting of the alimentary canal, even in otherwise axial types 

 like the limpet. These constitute the group Aspidobranchia. 

 In them antero-dorsal respiratory gills evolved with an evolv- 

 ing heart. 



Second, and perhaps simultaneously, a series that retained 

 straight alimentary canal, that retained 4 or only 2 tentacles 

 and a well-developed pair of excretory tubes but that became 

 monoecious, split up into one set which evidently assumed 

 an increasingly land habit, evolved an anterodorsal pulmonary 

 sac, formed no operculum, and either added to the chitinous 

 l)rimitive shell a light calcareous one, or absorbed the latter 

 and became shell-less in the adult. These constituted the 

 Pnlmonata. Another set that seems to have migrated sea- 

 ward developed in many cases a heavy calcareous shell, formed 

 a pre-anal and post-dorsal branchial expanse, and so constituted 

 the Opisthobranchiata. 



Third, and apparently derived from both divisions of the 

 first by later and condensing modification, arose two parallel 



