506 



Causes and Course of Organic Evolution 



turn early evolved from fresh-water types is a question on 

 which no light has yet been shed. 



A point is now reached where the palseontological and the 

 zoological records are equally defective. For granting the 

 existence of archaean organisms that formed from the single 

 egg cell a morula, or cellular embryonic mass, as in young 

 phases of the Spongida, Coelenterata, and Echinodermata, 

 few organisms seem to exist now which connect such with 



soc 



-mc 



Fig. 20 — Diagram of Rotifer after Zelinka: a-p apical plate or area; 6p basal 

 do.; 80C. anterior ciliated ring; 8ac posterior do.; m mouth; a anus of alimentary 

 canal; db dorsal brain or cerebral ganglion; vh ventral do.; e eyes; nep nephri- 

 dia; eg cement glands. 



those higher and more complex alliances that evidently rep- 

 resent the main line, as well as many collateral or diverging 

 lines of ascent, like the Rotifera, the Turbellaria, and the 

 Polyzoa, not to name higher classes. 



But larval or developmental stages of the higher classes 

 seem clearly to point the way backward toward the Rotifera 

 as a great basic group, from which have sprung most of the 

 dominant classes now existing. As leading up to this it may 

 be said that zoologists are now largely agreed in regarding the 

 so-called trochophore stage of many large phyla or branches 

 of invertebrate animals as representative temporarily of what 



