ON THE DIPLOCHOIIDA. 339 



Figs. 33 — 36. — Series of senii-diagranimatic figures illustrating the pro- 

 cess of vacuolisatiou. 



Fig. 37. — Oblique section of uotochord in young Plioronis. 



Fig. 38. — Side view of front parts of fully developed Actinotrocha. 



Fig. 39. — Transverse section of same larva as Figs. G — 17, in trunk 

 region. 



Fig, 40. — Longitudinal section of collar nepliridium, as reconstructed 

 from serial sections. 



Fig. 41. — Transverse section of duct of collar nepliridium. 



Fig. 42. — Transverse section of nerve ganglion and subneural sinus. 



Fig. 43.— Ditto of tentacle (larval). 



Fig. 44. — Ditto of pre-oral nerve-ring. 



Fig. 45. — Longitudinal section of liind end of larva to show the doubtful 

 organs. 



PLATE 22. 



Fig. 46. — Semi-diagrammatic dorsal view of nervous system of Balano- 

 glossus. 



Fig. 47. — Ditto of Actinotrocha. 



Fig. 48. — Scnii-diagramniatic median sagittal section of front end of 

 Balanoglossus. 



Fig. 49.— Ditto of Actinotrocha. 



Fig. 50. — Semi-diagrammatic median coronal section of Balanoglossus. 



Fig. 51. — Ditto of Actinotrocha. 



List of Abbreviations. 



b. c' Pre-oral body-cavity, b. c? Trunk body-cavity, b. s. Blood-sinus. 

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

 ep. Epistome. (/. s. Gill-slit. int. Intestine. /. b. v. Lateral blood-vessel. 

 I. c. c. Left collar cavity, m. Mouth, n.g. Nerve ganglion, n. Nuclei. 

 nch. Notochord. ov. Ovum. j). o. I. Post-oral lamella. ph. Pharynx. 

 p. p. Proboscis-pore. r. c. c. Eight collar cavity, st. Stomach. &h. Sheath. 

 *. n. s. Subneural sinus. s. n. g. Subneural gland. t. Tentacle. v. b. v. 

 Ventral blood-vessel, v. mes. Ventral mesentery. 



