Evolution of Animals 393 



back to the period of the upper cambrian, or the ordovician. 

 So there must certainly have been a period long antecedent 

 to the later cambrian when molluscs existed as adult animals 

 in. the "veliger" stage, but were modifying the cement or shell 

 gland that hereditarily was common to them, to the Rotifera, 

 and to the Brachiopoda mto a shell secreting organ. For 

 long ages however the group must have existed as soft-bodied 

 animals only, and the geologic period between the origin of 

 veliger cephalopods and the origin of orthoceratite genera 

 with shells a foot to six feet long, as encountered in cambrian 

 and Silurian rocks, must have been an enormously extended one. 



So one is compelled to assume that not merely the common 

 ancestral types of the group, but these, along with other organ- 

 isms that linked such ancestral forms with other phyla of the 

 animal kingdom, had developed during the early cambrian 

 or more likely even well back into late-archsean times. Now, 

 if successive embiyological stages, and the principle of devel- 

 opmental recapitulation, are of any value, both indicate that 

 in the trochosphere, and later in the veliger stages of molluscs, 

 direct contact is made with the Rotifera, as well as ^dth tur- 

 bellarian and nemertean ancestors, as several zoologists have 

 already emphasized. But the body-substance alike of a roti- 

 fer, a turbellarian, a nemertean and of a veliger mollusc is 

 extremely soft. It is therefore not surprising that their fossil- 

 ized remains are unrecorded in rocks of the early cambrian 

 or of precambrian age. But proofs have been adduced m 

 favor of the view (pp. 387, 389) that the Rotifera and the 

 Nemertinea both originated in lacustrine areas. 



So, in late-archsean times, ancestors of the rotifers, the 

 turbellarians, the nemertines, and post-viliger ancestors of 

 the molluscs probably all lived side by side in such waters. 

 The frequently saline character of some lacustrine areas, into 

 which dissolved salts were carried, formed a ready natural 

 source from which to draw calcareous shell-constituents as 

 well as to accustom such organisms to a subsequent marine 

 life. Now at the present day a considerable number of mol- 

 luscan genera and species show marked capacity of adapta- 



13* 



