ON THE DIPLOOHORDA. 335 



primitive character, whilst the latter exhibits extreme degenera- 

 tion in the adult. 



The main chordate stem then underwent raetameric segmen- 

 tation, and gave rise to the En -chord a. Of these, those 

 still retaining an active pelagic or littoral life gave rise to the 

 Vertebrata or Holochorda (Gadow) ; whilst others again, 

 falling out of the ways of progress, sufFere d the same fate as the 

 Hemichorda and Diplochorda. The Cephalochorda, 

 taking to a burrowing habitat, suffered like modifications (such 

 as pre-oral extension of the notochord) to the Hemichorda ; 

 whilst the Urochorda adopting the sedentary habits of the 

 Diplochorda, and especially those of Phoronis, like this 

 group suffered extreme degeneration. The metameric segmen- 

 tation of the Urochorda is not beyond dispute, but the con- 

 sensus of workers on the group appears in favour of regarding 

 them as metamerically segmented, at least primarily.^ The 

 views here propounded may be expressed in this classification. 



Chordata. 



I. Archichorda. 



Archimeric segmentation into protomere, paired mesomeres, 

 andmetamere; little or no metameric segmentation. Noto- 

 chord in primitive continuity with the walls of the gut 

 throughout life. More or less connected with the protomere, 

 the main animal organ of the body. Nerve-ganglion between 

 protomere and raesomere or dorsal tomesomere. Main nerves 

 are protomeric ring, mesomeric ring, and dorsal and ventral 

 trunks. A mesoblastic choudroid skeleton and an ectodermal 

 chitinoid tube or skeleton. 



1. Hemichorda. — Notochord fused in middle line and 

 protruding far into the protomere. Commencing metameric 

 segmentation in gill-slits and gonads. 



2. Diplochorda. — Notochord in primitive paired condi- 

 tion. In close connection with the two posterior protomeric 



' The position of Urochorda is undoubtedly nearer the Eu-chorda than 

 the Archichorda. 



VOL. 40, PART 2. — NEV7 8ER. A A 



