CONCLUSION. 291 



in their evolution and the more remote their genetic 

 affinity. 



For the present we may conclude that the proximate 

 ancestor of the Vertebrates was a free-swimming animal 

 intermediate in organisation between an Ascidian tadpole 

 and Amphioxus, possessing the dorsal mouth, hypophysis, 

 and restricted notochord of the former ; and the myo- 

 tomes, coelomic epithelium, and straight alimentary canal 

 of the latter. The ultimate or primordial ancestor of the 

 Vertebrates would, on the contrary, be a worm-like animal 

 whose organisation was approximately on a level with 

 that of the bilateral ancestors of the Echinoderms. 



NOTES. 



i. (p. 246.) For the discussion of the phenomena of meta- 

 merism and the enumeration of examples of independent metameric 

 repetition of parts, consult the following : LANG, ARNOLD. Der 

 Ban von Gunda Segmentata und die Verwandtschaft der Plathel- 

 minthen mit Ccelenteraten und Hirudineen. Mitth. Zool. Stat. 

 Neapel, Bd. III. 1882. p. iS-j e/ seg. SEDGWICK, ADAM. On 

 the Origin of Metameric Segmentation, and Some Other Mor- 

 phological Questions. Quarterly Jour. Micro. Sc. XXIV. 1884. 

 pp. 43-82. BATESON, WILLIAM. The Ancestry of the Chordata. 

 Quarterly Jour. Micro. Sc. XXVI. 1886. pp. 535-571- CALD- 

 WELL, H. Blastopore, Mesoderm, and Metameric Segmentation. 

 Quarterly Jour. Micro. Sc. XXV. 1885. pp. 15-28. HUBRECHT, 

 A. A. W. Report on the Nemertea collected by H. M. S. Challenger, 

 1873-76. Chall. Kept. Zool. XIX. 1886. (Also, HUBRECHT. 

 The Relation of the Nemertea to the Vertebrata. Quarterly Jour. 

 Micro. Sc. XXVII. 1887. pp. 605-644.) VAN BENEDEN, 

 EDOUARD. Recherches sur le Dweloppement des Arachnactis. 

 Contribution a la Morphologie des Cerianthides. Archives de 

 Biologic, XI. 1891. pp. 115-146. Also consult the recent 

 great work of BATESON, Materials for the Study of Variation. 

 London, 1894. 



