366 A. T. MASTERMAN. 



Fig, 19. — Transverse section of lateral notochord, passing to pharyngeal 

 cleft on the left and pharyngeal wall on the right. 



PLATE 26. 



Fig. 20. — Diagrammatic median sagittal section of front part of Cepha- 

 lodiscus. 



Fig. 21. — Diagrammatic transverse section of pharynx, of Actinotrocha. 



Fig. 22. — Ditto of Cephalodiscus. 



Fig. 23. — Ditto of Balanoglossus. 



Fig. 24. — Diagrammatic transverse section of stage in evolution of Verte- 

 brate (Eu-chordate) pharynx. 



Fig. 25. — Ditto of pharynx of Eu-chordata. 



Fig. 26. — Transverse section of a plume of Cephalodiscus. 



Fig. 27. — Ditto of a pinna of Cephalodiscus. 



Figs. 28, 29. — Refractive globules isolated from the " eyes." 



Fig. 30. — Longitudinal section of branchial eye of Cephalodiscus. 



Fig. 31. — Ditto of single cell from branchial eye of Cephalodiscus. 



Figs. 32 — 36. — Serial transverse sections of plumes and collar region. 



Abbeeviations. 



b.c.l. Coelomic cavity of epistome. b.c.Z. Trunk ccelomic cavity, h.s. 

 Blood-sinus, c.p. Collar-pore, d.b.v. Dorsal blood-vessel, d.mes. Dorsal 

 mesentery, ep. Epistome. gd. Gonaduct. g.s. Gill-slit (pharyngeal cleft). 

 hit. Intestine. l. b. v. Lateral blood-vessel. /. c. c. Left collar-cavity 

 (ccelomic). l.n. Lateral nerve, m. Mouth, nch. Notochord. n.g. Nerve 

 ganglion. 7i. Nucleus, o.g. Oral grooves, ov. Ovary, ph. Pharynx. 

 p.o.l. Post-oral lamella, p.o.n. Pre-oral nerve, r.c.c. Right collar cavity 

 (ccelomic). sh. Sheath of notochord. st. Stomach, s.n.g. Subneural gland. 

 s.n.s. Subneural sinus, t. Plume. 



