595.2 264 



V. 



Are the Arthropoda a Natural Group ? 



A NATURAL group from the stand-point of genealogical zoology is 

 a group the branches of which all start from a common stem or 

 ancestral stock which, while giving rise to them, grouped and con- 

 nected at their origin with one another, gives rise to no other line of 

 descent. 



The Arthropoda appear to me to form such a natural group. 

 Their ancestral stock, the Pro-Arthropoda, joins with the ancestral 

 stocks of the Chaetopoda and Rotifera, forming the large natural group 

 of the Appendiculata. I urged this connection and proposed the term 

 Appendiculata in 1878 in the preface to the English edition of 

 Gegenbaur's Elements of Comparative Anatomy. At that time 

 German zoologists placed the Chaetopoda in the group Vermes and 

 cut them off from all association with the Arthropoda. Huxley on 

 the other hand had adopted Macleay's term Annulosa for a group 

 including Chaetopoda and Arthropoda but excluding Rotifera. 



By associating the Chaetopoda with the Arthropoda the parapodia 

 of the former were identified with the appendages of the latter, and it 

 became necessary to interpret the head of the arthropod in relation 

 to the prostomium and peristomium of the chaetopod. 



I suggested in 1872 that "the ophthalmic segment alone in 

 Arthropoda represents the prostomium [of Chaetopoda] , the 

 antennary and antennular segments being aboriginally metastomial 

 and only prostomial by later adaptational shifting of the oral 

 aperture." 



Moseley's discovery soon after this of the parapodial nature of 

 the jaws of Peripatus, and the later discovery of the paired nephridia 

 in every segment of its body, strengthened the hypothesis of a genetic 

 connection between chaetopods and arthropods. 



In 1881 my study oi Limulus, Scorpio, and Apus led me to the view 

 that the prostomial antennae of the chaetopod are retained by Peripatus, 

 the Myriopoda, and Hexapoda, but have been entirely lost by 

 Crustacea and Arachnida, whence I proposed to call the former group 

 Ceratophora and the latter Acerata. 



The latest results of embryological investigation have shown that 

 this view is probably incorrect, and according to Professors Korschelt 

 and Heider a large and important application of my theory of the 



