Fig. 18 — a dorsal, 6 ventral view, brain of Ewpolia Giardii (after 

 Hubrecht) ; c, d, of hypothetical protochordate type; e, f, of cyclos- 

 tome type; ol. n., ol. a., rh., olfactory nerves and area or rhinencepha- 

 lon; op. 71., op. a., optic nerves (which should be yellow in all) and area; 

 th., the last evolved as thalamencephalon ; p. c, posterior commissure; 

 (i.?i., dorsal nerve becoming Reissner's fibre in c-f; v. c, ventral com- 

 missure; n:e., mid lobe or mesencephalon; n. s., auditory sac; ??i. y, 

 ventral ganglia becoming myelencephalon or medulla in c-f; In, lateral 

 nerves becoming sp. c. or spinal cord in c-f. 



