330 A. T. MASTERMAN. 



his hypothesis^ and put no claim to originality in this par- 

 ticular point. 



If the metameric or secondary segmentation took place 

 between the stages of Balanoglossus and Amphioxus it is 

 evident that^ as Bateson has emphasised, the '' segmented " 

 Invertebrata and the segmented Vertebrata must be 

 genetically distinct^ in spite of the most elaborate anatomical 

 resemblances. The recognition of this archimeric segmenta- 

 tion and its relationships to the secondary or " metameric'^ 

 segmentation will, I venture to think, not only clear up many 

 difficulties in tracing the origin of Vertebrata, but will 

 furnish a sound basis for the phyletic classification of the 

 Coelomata. 



Although the archimeric segmentation would form the basis 

 of the group Archicoelomata, here suggested, a primitive 

 condition of the other systems is characteristic. Thus we might 

 state the following characters of this group : 



1. The body is divided into three archimeric segments, the 

 protomere, the paired mesomeres, and the metamere, either 

 throughout life or in early stages,^ when the relative promi- 

 nence of one or more of these may be altered. 



2. The metamere is never definitely divided up by a secondary 

 bilateral segmentation, usually known as ''metameric^' seg- 

 mentation. 



3. The nervous system is usually in a primitive condition, 

 and is still in continuity with the ectoderm throughout life. 



' The coronal section of Bateson's early larva of Balanoglossus clearly 

 shows indications of primitive equivalent radial value of the four pouches 

 though the niesomeric elongate very early. In ontogeny one need not 

 expect the four pouches to, in every type, be separately invaginated from the 

 gut. A diiference in the relative time of development of different parts would 

 readily account for the variations in this respect seen in the different groups 

 of Echinodermata and the Chaetognatha, though the separate invagina- 

 tion of each from the hypoblast must be regarded as the more primitive con- 

 dition, just as the elongation of blastopore to join both mouth and anus is 

 regarded by many as the more primitive condition, and the survival of this 

 aperture as the anus or mouth only is conceded to be an ontogenetic 

 adaptation. 



