ILLUSTKAT10NS xix 



65. The Zoanthina larva of a Zoantharian . . 86 



66. Transverse sections of two types of Actinozoan 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 Caryophyliiu, ci/nl/nts . 88 



69. Side view of the segmenting egg of a Ctenophore (Oallianira Inal/i/n} . 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 (Jteiiophore 



embryo (Collianira Malata) ...... 91 



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



endodermal cavities ....... 92 



74. Optical section of embryo of Beroe forskiilii 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 . 9-4 



78. The free-swimming larva ofCallianira Malata, 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 inquiiina. Eight-cell stage, viewed from 



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



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



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



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



vegetative pole ........ 107 



83. Optical section of the developing egg of Planocera inqitiii/tx, viewed from 



posterior pole ..... . 108 



84. Diagrammatic frontal section through the egg of Planocera inqnili/ia at a 



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



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



from animal pole ........ HO 



86. Three longitudinal sections through developing embryos of PtoioceroMigwJmo! Ill 



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



aurantiaca . . . . . . . . .112 



88. Lateral view of the free-swimming larva of Yumjia aurantiucu . . 112 



89. A, dorsal, and B, ventral views of larva of Yu-iujla aurantiaca in which 



metamorphosis is beginning . . . . . .113 



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



metamorphosis is almost complete ..... 113 



91. Median sagittal longitudinal section through larva of Yunyia auratitiaca . 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 Ccrebratulus lacteus, viewed 



from the side ... . .119 



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



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



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



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



96. Two views of advanced Pilidium larva of L'crcbrattthis lactcus in order to 



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



