ON THE DIPLOCHORDA. 413 



of Actinotrocha and Bala oglossus points out clearly 

 that choi'doid modification may occur in any part of the 

 hypoblastic organs. On the other hand, in Cephalo- 

 discus, and, as I hope to show elsewhere, in Tornaria, the 

 chordoid metamorphosis is restricted to a pair of lateral 

 grooves along the pharynx, so that we are justified in sup- 

 posing that this represents the earliest condition of the 

 Archichorda, and that the Diplochordate condition has 

 become disguised in adult Balanoglossus by a further 

 spreading of the chordoid metamorphosis. Hence in 

 Balauoglossus there does not appear to be any one 

 organ which can be compared directly to the noto- 

 chord of the Eu chord a. 



The phyletic history of an organ appears to consist of the 

 following steps : 



1. The function and structure are co-extensive with the 

 organism itself, or very early limited to either the endoderm, 

 ectoderm, or mesoderm. 



2. The function becomes concentrated in a certain part 

 of the primary layer, and the part itself then becomes differ- 

 entiated structurally from the rest of the layer. 



3. The function is so fully defined as apart from that of 

 the primary layer as a whole, that the organ also becomes 

 organically separated fi'om the parent layer. In a natural 

 group it is to be expected that the lower members will 

 exhibit stages 1 and 2 of organs which are in stage 3 in the 

 higher. 



It has been suggested to divide the Chordata into two 

 main groups, Archichorda and Euchorda, the former com- 

 prising Balanoglossus, Cephalodiscus, Phoronis, and 

 possibly the Echinodermata ; whereas in the latter there are 

 included the Urochorda, Cephalochorda, and Vertebrata. 



The Archichorda are regarded as being the lower 

 members of the group, and we may inquire. How do they 

 stand the test of the organismal relationships above referred 

 to? 



In the case of the nervous system, stages 1 and 2 are 



