150 



THE AMK, 



■A LI ST 



[Vol. LIII 



more features in common with the lower arthropods than 

 have such primitive forms as the Rotifera. On the other 

 hand, the Annehda themselves (and hence ultimately the 

 Arthropoda also) are the descendants of Eotif era-like 

 (and Platyhelmintlies-like) forebears, and it is quite 

 possible that certain rotifers might develop features 

 which later find opportunity for fuller expression in the 

 forms descended from them (e. g., the striated muscles 

 of Pedalion ) ; but, since the general organization of a 

 rotifer's body is not so similar to that of the lower 

 arthropods as is the case in the annelids in question, for 

 the present at least it seems preferable to regard the 

 slight resemblances between the appendages of the Rotif- 

 era and the lower Arthropoda mentioned above as the 

 result of ''convergence" (parallelism) in development 

 rather than to consider it as a precocious development of 

 structures later to be developed in the arthropod de- 

 scendants of ancestors ultimately arising from rotiferan 

 forebears. I would not utterly deny the latter possibility, 

 however, since it may be quite possible that arthropods 

 are to be derived more directly from Rotifera-like fore- 

 bears (e. g., Hexarthra polyptera, etc.) through forms 

 related to the Tardigrada and Nauplius-\\ke ancestors; 

 but the great mass of evidence from comparative anat- 

 omy, embryolog}^, etc., points to an ''annelid ancestry" 

 for the Arthropoda, and until other hitherto undiscov- 

 ered forms have been found to indicate some other deri- 

 vation for the group, we are safe in assuming that the 

 "Annelida" represent as nearly as any known forms the 

 ancestral condition of the Arthropoda. 



In taking up the consideration of the evolution of the 

 Arthropoda themselves, the question naturally arises as 

 to what arthropods liave departed the least from the 

 probable ancestral condition of the group as a whole. 

 Some investigators would claim that since the Nauplius 

 larva is of such widespread occurrence among the lower 

 arthropods, that it represents an ancestral type; but it 

 must be borne in mind that a free-swimming larva is 



