coxcLUSioy. 2gr 



in their evolution and the more remote their g;enetic 

 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, qn 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 follownng : Lang, Arxold. Der 

 Bail von Gunda Segmentata und die Verwandischaft der Plathel- 

 minthen mii Cmlenteraten und Hiriidineen. Mitth. Zool. Stat. 

 Neapel, Bd. III. 1SS2. p. iS-j et seq. Sedgwick, Ad.am. On 

 the Origin of Metameric Segmentation, and Some Other J\for- 

 phological Questions. Quarterly Jour. Micro. Sc. XXIV. 1SS4. 

 pp. 43-82. B.iTESON, W1LLLA.M. TJie Ancestry of the Chordata. 

 Quarterly Jour. Micro. Sc. XXAT. 1SS6. PP- 535-571- Cald- 

 well, H. Blastopore, Alesoderm, and Metameric Segmentation. 

 Quarterly Jour. Micro. Sc. XXA^. 1SS5. pp. 15-28. Huerecht. 

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

 1873-76. Chall. Rept. Zool. XIX. 1S86. (Also, Hubrecht. 

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

 Micro. Sc. XXVII. 1887. pp. 605-644.) Vvv Bexedex. 

 Edou.\rd. Recherches sur le Developpement des Arachnactis. 

 Contribution a la Morphologic des Cerianthides. Archi\'e3 de 

 Biologie, XL 1891. pp. 115-146. Also consult the recent 

 great work of B.iTESOX, Materials for the Study of Variation. 

 London, 1S94. 



