520 Causes and Course of Organic Evolution 



The above study indicates that, if the existing preponderating 

 abundance of rotifers in fresh water is a true key to past con- 

 ditions, it seems Hkely that the brachiopods sprang from a 

 primitively lacustrine rotiferan ancestry, and that through 

 loricate members of the Rotifera, which migrated seaward 

 during later archsean times, loricate and afterward shelled 

 brachiopods evolved, that even, in early caml)rian times were 

 abundant marine organisms. Such a view furnishes at least 

 a working hypothesis that further embryological study may 

 strengthen or weaken. 



The phylogenetic origin of the Mollusca may next be ex- 

 amined. For these as for some preceding groups, not a few 

 zoologists have claimed an origin common with the Polyzoa 

 and Annelida, more rarely with the Rotifera. The last seems 

 perfectly to furnish the fundamental starting point, and, as 

 Korschelt-Heider (171, 1: 260) have emphasized, some adult 

 rotiferan groups supply strong points of contact with mol- 

 luscan larvae. But, as the Mollusca now exist, we are at once 

 confronted with widely different larval types that doubtless 

 represent equally divergent and early diverging adaptational 

 modifications to environal relations. 



As in other of the rotifer-derived groups already studied 

 the ecto- and endoderm of the trochophore larva may arise 

 either by epiboly or emboly. Except in the most highly modi- 

 fied group of the Cephalopoda an embryonic stage is reached 

 by molluscs that succeeds to the simple trochophore,* and 

 has been called the veliger stage. It is in the earlier or in the 

 mid period of veliger development that perfect homologies 

 to the rotifers are best observed; while, from this period on, 

 localized growth activities are then ushered in — much as with 

 the Polyzoa and Brachiopoda already reviewed — that can 

 best be interpreted in terms of continued rotifer-molluscan 

 evolution. 



* In the above connection J. T. Cunningham has recently written "most 

 remarkable is" the trochophore's "resemblance to the adult form of the Wheel 

 Animalcules or Rotifera, which retain the preoral ciliated band as their chief 

 organ of locomotion and prehension throughout life." 



