ON THE DIPLOCHORDA. 329 



We note that the anus has travelled to the posterior eud^ 

 in obedience to general laws of symmetry. 



This change from radial to bilateral symmetry, though 

 hypothetical, is paralleled in a remarkable manner in the 

 Echinoids, and the above woodcuts will forcibly recall the 

 transitional symmetry of such forms as Spatangus. 



The change in position of the anus is important, for it 

 places the metameres (3, 3'), now separated by the alimentary 

 canal into two, in an advantageous position in regard to 

 the processes of nutrition, and hence we find that not only 

 do these metameric coelomic cavities early monopolise the 

 reproductive function to the exclusion of the protomere and 

 the mesomeres, but they elongate enormously ; and in them 

 is instituted in higher forms a secondary bilateral seer- 

 mentation, which is already termed '^metameric." The 

 study of the segmented forms leads one to believe that the 

 metameric segments grow forward, at least the coelomic 

 elements in them, and to a large extent replace the archimeric 

 protomere, and mesomeres. 



The stage we have now reached is practically identical with 

 that exhibited by Actinotrocha (without taking into con- 

 sideration the notochordal rudiments). I need not go into a 

 detailed comparison of the two types, — Actinotrocha is so 

 closely represented by this stage that its study suggested the 

 hypothesis here laid down. Woodcuts 12, 13, and 14 would 

 almost pass for diagrammatic representations of Actino- 

 trocha. These are the bare outlines of a suggested ancestry of 

 the Chordata, tracing the lines of evolution nearly to the 

 Coelenterata. We must assume that the archimeric sesf- 

 mentation is predominant till above the Hemichorda, 

 and that in Amphioxus the metameric segmentation comes 

 into existence, and the protomere and mesomeres become 

 of secondary importance or disappear altogether. I find, in 

 looking at the literature of Balanoglossus, that Morgan 

 has suggested that the condition of the mesoderm in Am- 

 phioxus is arrived at through secondary segmeutatiuu 

 of the trunk of Balanoglossus, so that I must adopt 



