EXPLANATION OF PLATES IT & III— continued. 



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

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

 P. Poucbes of arcbenteron (four represented on eitber side). C. Longitu- 

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

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



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

 separate from central part of arcbenteron (nowcalled mesenteron). jE'.Ectoderm. 

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

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

 part of primitive mouth. N. Central nervous system ; dumb-bell sbaped 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 Inveitebrata). Excretory pores are not developed, except behind, 

 A. P. ; and in front, 6. Colours and letters as in Fig. 8, except G, gill 

 pouch. K. D. 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. 

 K. Nephridia. 



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

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

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



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

 cular system, red. P. Prseoral lobe (iisemal). H. Heart. N. Nervous 

 system. W. 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 prajoral 

 lobe and without praeoral 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. 



