ILLUSTKAT10NS xix 



f'lG. PAGE 



65. The Zoauthina larva of a Zoanthavian . . . . .86 



66. Transverse sections of two types of Actiuozoaii larvae . . .86 



67. Young living Caryophyllia cyathus, seen from above. The calcareous 



skeleton shows through the transparent tissue . . . .87 



68. Five stages in the development of the skeleton of Caryophyll-ia cyathus . 88 



69. Side view of the segmenting egg of a Ctenophore (Callianira bialata) . 89 



70. Two views of the developing egg of Beroe ovata, seen from above . . 90 



71. Oral and aboral views of the embryo of Beroe ovata in a later stage of 



development ........ 90 



72. Illustrating the origin and fate of the so-called mesoderm in a Ctenophore 



embryo (Collianira bialata) ...... 91 



73. Optical section of embryo of Beroe forskalii showing the beginning of the 



endodermal cavities ....... 92 



74. Optical section of embryo of Beroe forskalii in a later stage of development, 



with a hollow endodermal sac ...... 92 



75. Two optical sections through the embryo of Beroe forskalii . . . 93 



76. Larva of Beroe forskalii four days old, viewed from "stomach-plane" . 93 



77. Part of apical region of larva of Beroe forskalii, viewed from stomach-plane . 94 



78. The free-swimming larva of Callianira bialata, viewed from the stomach-plane 95 



79. An embryo of Beroe ovata with four ribs and two endodermal pouches, and 



a small extra third pouch ; obtained by isolating one of the first two 

 blastomeres of Beroe ovata ....... 97 



80. Developing egg of Planocera inquilina. Eight-cell stage, viewed from 



animal pole ......... 105 



81. Developing egg of Planocera inquilina. Sixteen-cell stage, viewed from the 



animal pole ......... 105 



82. Developing egg of Planocera inquilina. Thirty-two-cell stage, viewed from 



vegetative pole ........ 107 



83. Optical section of the developing egg of Planocera inquilina, viewed from 



posterior pole ........ 108 



84. Diagrammatic frontal section through the egg of Planocera inquilina at a 



later stage of development than that represented in Fig. 83 . . 109 



85. Developing egg of Planocera inquilina in a late stage of segmentation, viewed 



from animal pole ........ 110 



86. Three longitudinal sections through developingenibryosofPlanocerainquilina 111 



87. A, dorsal, and B, ventral views of the free-swimming larva of Yungia 



aurantiaca ......... 112 



88. Lateral view of the free-swimming larva of Yungia aurantiaca . . 112 



89. A, dorsal, and B, ventral views of larva of Yungia aurantiaca in which 



metamorphosis is beginning . . . . . .113 



90. A, dorsal, and B, ventral views of larva of Yunyia aurantiaca in which 



metamorphosis is almost complete ...... 113 



91. Median sagittal longitudinal section through larva of Yungia aurantiaca . 114 



92. Median sagittal section through a young Polyclade worm ( Yungia aur- 



antiaca) just after its metamorphosis ..... 115 



93. Two stages in the segmentation of the egg of Cerebratulus lacteus, viewed 



from the side ... . . . . . . 119 



94. The young gastrula of Cerebratulus lacteus ..... 120 



95. Two stages in the development of the Pilidium larva of Cerebratulus lacteus 



showing the development of mesenchyme into muscles. A, earlier stage. 



B, later stage ........ 121 



96. Two views of advanced Pilidium larva of Cerebratulus lacteus in order to 



show the development of the muscles ..... 121 



