The Development of Phascolosoma. \(^\ 



Fig, 77. Larva, thirty days, with the posterior part of the body in 

 a somewhat contracted condition. 230: 1. 



Fig. 78. Larva, nineteen days, with the introvert retracted, showing 

 epidermal papillae, circular muscles, reproductive cells, etc. Drawn from 

 a preparation. 230 : 1. 



Fig. 79. Yolk granules from the coelom of a young larva of sixty- 

 five hours. 900 : 1. 



Fig. 80. a. Coelomic corpuscles (nuclei of prototroch cells) from a 

 larva of eighty hours, b. Coelomic corpuscles from a larva of six and a 

 half days. c. Small coelomic corpuscles from a larva of thirteen days. 

 Magnification of all is 900 : L 



Fig. 81. Corpuscles from the coelom of an adult Phascolosoma, for 

 a comparison of the size with that of the larval corpuscles shown in 

 Fig. 80. 900 :L 



Plate 10. 



Sections of the trochophores of Ph. vulgare and of Ph. gouldii. 

 Fig. 82 — 84, Series of cross sections of the trochophore of Ph. vulgare, 

 forty-six and a half hours old. 455 : 1. 



Fig. 82. Cross section through the region of the stomodaeum and 

 of the prototroch cells. 



Fig. 83. Cross section through the region of the postoral circlet, 

 showing nuclei of the retractor muscles lying in groups within the coelom, 

 the ventral nerve cord, etc. 



Fig, 84. Cross section through the region of the trunk. 



Fig. 85, Tangential section, showing the superficial cells in the 

 equatorial region in a trochophore of Ph. vulgare of forty- four and a half 

 hours. Behind the large yolk-laden cells of the prototroch are shown the 

 two parts of the middle sphincter muscle, between which lies a row of 

 cells which bear the postoral circlet of cilia. 830 : 1. 



Fig. 86. Nearly sagittal section of an elongated trochophore of Ph. 

 gouldii, fifty-one hours old, showing the ruptured yolk membrane, cuticula, 

 segmented rudiment of the ventral nerve cord, etc. 455: 1. 



Fig. 87. Parasagittal section of a slightly older trochophore {Ph. 

 gouldii, 57 hours) showing segmentation of the coelomesoblast, etc. 455:1. 



Fig. 88. Parasagittal section of the same embryo, showing the 

 degenerating prototroch cells, retractor muscles, etc. This is a combination 

 of two sections. 455 : 1. 



Fig. 89. Sagittal section of the same trochophore (57 hours), 

 showing the rudiments of the ventral nerve cord, proctodaeum, etc. 455: 1. 



Plate 11. 



Fig. 90. Obliquely frontal section of the prostomium of the larva 

 of Ph. vulgare, sixty-three hours old (before the outgrowth of the lateral 



