Phylogeny of x\niivials 509 



We have thus recapitulated details such as can be got in 

 any zoological manual, in order later the more readily to refer 

 intelligently to minuter details that genera or species show, 

 and which may indicate points of contact with other, higher, 

 and, as we would view it, derived groups. 



With such details before us we believe that evolutionary 

 progression and descent can be readily traced to the Tur- 

 bellaria, the Nemertinea, the Annelida, the Polyzoa, the Brach- 

 iopoda, and the Mollusca, while we will refer finally to clear 

 affinities with the Arthropoda, such as Gosse claimed fully 

 a quarter century ago. In other words, the Rotifera present 

 us with a highly generalized and primitive group from which 

 all of the above may be derived, and which indicate their 

 ancestry by the temporary larval rotifer-like trochophore or 

 veliger stage that they still pass through. 



But of the above large groups the one that, in its smallest 

 and simplest members, shows close affinity to the Rotifera, 

 though in its largest and most complicated genera advances 

 far beyond these, is that of the Turbellaria. Comparison 

 of the structure of the smaller rhabdocoel turbellarians with 

 the Rotifera and with the trochophore of higher classes sug- 

 gests to us what may have been a common and simpler type 

 from which all have arisen, or in other words, a primitive rotifer. 

 This was in all probability holociliate, faintly divisible into 

 an apical, a median, and a basal region, had a simple blind 

 alimentary canal (as have many male rotifers still, though 

 owing apparently to degeneration), without or to a slight 

 extent with accessory glands, with simple eye-spots that were 

 formed on or near epidermal invaginations that became at 

 first separate and later fused ganglia, while outgrowths formed 

 from the last that became nerve threads. 



Every detail of this composite picture can even now be 

 followed in rotifers or in fresh-water rhabdocoel turbellarians. 

 But all known analogy suggests that during early epochs, 

 and most abundantly probably during later mid-archsean 

 times, organisms swarmed in the warm stimulating fresli waters 

 of that period that were similar to the above, and which con- 

 tributed gradually to the evolution of higher types than they. 



