The Development of Phascolosoma. ;[59 



Fig. 46. View of the anterior hemisphere of a trochophore of thirty- 

 three hours, showing structures which appear at a deep focus, viz, dorsal 

 cord, pigment spots, apical sense organ, stomadaeum, etc. 280 : 1. 



Fig. 47. Similar view of a slightly older embryo (thirty-six hours), 

 showing the nineteen cells of the prototroch, the apical plate, definitive 

 rosette and dorsal cord of ectoderm in the interruption of the prototroch. 

 Drawn from a preparation. 300 : 1. 



Fig. 48. Frontal section of a trochophore (thirty-nine hours) to show 

 the origin of the dorsal retractor muscles. 280: 1. 



Fig. 49. Ventro-lateral view of an embryo, forty-five hours old, 

 showing adorai and postoral cilia, etc. 300 : 1. 



Fig. 50. Ventral view of a trochophore about forty hours old. 300: 1. 



Fig. 51. Embryo of about fifty- three hours, undergoing metamorphosis 

 and in the act of casting off the yolk membrane (z. r). 300: 1, 



Fig. 52. Dorsal view of an embryo of forty-nine hours, undergoing 

 metamorphosis. The yolk membrane has been shed, and the prototroch 

 cells are degenerating; their substance is gradually being taken up by the 

 coelom in the form of yolk granules. 300: 1. 



Fig. 53. Similar view of a slightly older embryo in which the annular 

 enlargement, due to the prototroch has nearly disappeared, the cells of 

 the latter having degenerated completely, 300: 1. 



Fig. 54. Side view of a slightly older embryo (sixty hours). 280: 1. 



Fig. 55. Parasagittal section of a larva of sixty hours. 



Fig. 56. Side view of a larva three weeks old, showing supraoeso- 

 phageal ganglion, ventral nerve cord, lateral nerves, alimentary tube, re- 

 tractor and circular muscles, nephridium, surface papillae, prostomial lobes, 

 etc. 280: 1, r r , r , 



Fig. 57. Similar view of an older larva of five weeks and six days, 

 showing internal organization and hooks on the introvert. 280 : 1. . 



Fig. 57a. View of the mouth and lobes of the prostomium from in 

 front. 280 : 1. 



Fig. 58. Nephridium of a larva five weeks old, as seen in an optical 

 section of the wall of the body. 400 : 1. 



Plate 8. 



Trochophores and larvae of Ph. gouldii. Magnification 384: 1, except 

 that of Fig. 63, which is 1200: 1. All figures are from living specimens, 

 except Fig. 63. 



Fig. 59. Surface view of trochophore, twenty hours after fertilization 

 of the egg. No eye spots have yet appeared. 



Fig. 60. Side view of tiochophore 36 — 40 hours old, showing adorai 

 band of cilia, in front of which a preoral band is being formed, apical 

 flagella, stomadaeum, and one of the pigment spots. The cuticula is being 

 secreted beneath the striated yolk membrane. 



