EXPLANATION OF PLATES X & Xl-continued. 



Fig. 7. — Diagram of ideal ancestor of segmented animals, viewed as a 

 transparent object from the ventral surface. A. Central part of archenteron. 

 P. PoucLes of archenteron (four represented on either side). C. Longitu- 

 dinal canal connecting pouches. 0. Excretory pores. N. Nervous ring. 

 M, A. Dumb-bell shaped mouth. Ectoderm. 



Fig. 8. — Diagram showing Invertebrate arrangement. Archenteric pouches 

 separate from central part of archenteron (now called mesenteron). ^.Ectoderm. 

 M.E. Mesenteron. M. S. Mesoblastic somites. K. Nephridia. 0. External 

 openings of Nephridia. 31. Mouth. A. Anus. M. A. Coalesced medium 

 part of primitive mouth. N. Central nervous system ; dumb-bell shaped like 

 that of Peripatus. Wall of mesenteron, yellow. Mesoblastic somites, blue. 

 Nephridia, red. 



Fig. 9. — Diagram of Vertebrate arrangement from neural (dorsal, i.e. ven- 

 tral of Invertebrata). Excretory pores are not developed, except behind, 

 A, P. ; and in front, G. Colours and letters as in Fig. 8, except G, gill 

 pouch. K. B. Longitudinal duct of Nephridia, or segmental duct, opening 

 behind into mesenteron. A- P. Pore retaining Invertebrate arrangement= 

 abdominal pore. Mouth, anus, and nervous system not shown. 



Fig. 10. — Diagrammatic transverse section through Invertebrate. Colours 

 as before. M. S. Somite. M. E, Mesenteron. N. Nervous system. 

 E. Nephridia. 



Fig. 11. — Diagrammatic transverse section through Vertebrate. Colours 

 as before. N. C. Neural canal. M. E. Mesenteron. K. D. Segmental duct. 

 K. Segmental tube (nephridium). M. S. Somite. 



Fig. 12. — Diagram of longitudinal vertical section of Invertebrate. Vas- 

 cular system, red. P. Preeoral lobe (hsemal). H. Heart. N. Nervous 

 system. N^. Prseoral nervous system. N-. Postanal ditto. M. E. Mesen- 

 teron. M. Mouth. A. Anus. 



Fig. 13. — Diagram of ideal intermediate type, with terminal mouth and 

 anus. Letters and colours as in Fig. 12. 



Fig. 14. — Diagram of arrangement of Balanoglossus, with neural prseoral 

 lobe and without prseoral and postanal nervous system. 



Fig. 15. — Diagram of arrangement of embryo Ascidians and Vertebrata. 

 Nervous system folded in. (Siphon stage.) Pi/, Anterior opening of neural 

 canal (site of the pituitary body). Ne. Posterior ditto. N. C. Neural canal. 



Fig. 16. — Diagram of Vertebrate arrangement. C. H. Cerebral lobe. 



