xx INVERTEBRATA 



FIG. I'AOK 



97. A Pilidium larva shortly before its metamorphosis . . . .' 122 



98. Longitudinal section through a Pilidium larva of about the age of that 



represented in Fig. 97 . . . . . .123 



99. Two stages in the development of the Nemertine rudiment within the 



Pilidium, viewed from above . . . . . .124 



100. The Trochophore larva of Polyrjordius, viewed from the side . . 130 



101. Stages in development of the blastula of Polygordius seen in optical 



longitudinal section ....... 132 



102. Dorsal view of upper hemisphere of egg of Polygordius, in which seventy- 



six cells have been formed . . . . . . .134 



103. Three stages in the segmentation of the lower or vegetative surface of the 



egg of Polygordius ....... 136, 137 



104. Four views of the vegetative pole of the developing egg of Polygordius in 



order to show the processes of gastrulation and of the formation and 

 closure of the blastopore . . . ' . . . . 141 



105. Four diagrammatic transverse sections of lower part of young Trochophore 



larva to show the mode of closure of blastopore .... 142 



106. Optical sagittal section of young Trochophore after gastrulation is complete 143 



107. Later stage in the development of Polygordius appendiculatus, in which 



the " worm-body " is being formed by the growth of the trunk blastema . 145 



108. Longitudinal sections of stage represented in preceding figure in order 



to show details ........ 146 



109. Two sections of the anterior part of a larva of Polygordius appendiculatus 



in order to show the changes supervening on metamorphosis . . 149 



110. Polygordius appendiculatus immediately after metamorphosis . . 150 



111. Longitudinal sections of anterior portion of Polygordius appendiculatus 



immediately after metamorphosis in order to show details . . 151 



112. Late larva of Polygordius lacteus in optical frontal section . . . 152 



113. Figure illustrating the origin of the mother cells of the adult mesoderm in 



Eupomatus ......... 154 



114. Diagrammatic sagittal section of fully-grown Trochophore larva of Eupo- 



matus in order to show the relative position of the protonephridia and of 



the coelomic rudiment '....... 155 



115. Stage in the segmentation of the egg of Nereis limbata, viewed from above, 



showing a laeotropic spiral cleavage of the egg .... 155 



116. The free-swimming larva of Nereis limbata, three days old. A typical 



" Polytrochal " larva ....... 156 



117. Transverse section through the ventral part of an embryo of Criodrilus 



lacuum ......... 158 



118. Two longitudinal sections through the ventral portion of an embryo of 



Criodrilus lacuum . . . . . . . .159 



119. Two longitudinal sections of embryos of Nephelis milgaris . . . 160 



120. A fairly advanced embryo of Clepsine (Glossiphonia), seen from behind . 161 



121. Hinder view of a well-developed larva of Nephelis vulgaris . . 162 



122. Larva of Nephelis vulgaris, viewed from the side .... 162 



123. Stages in the segmentation and the gastrulation of the egg of Peripatus 



capensis ....... . 170 



124. Stages in the division of the blastopore and the formation of the mesoderm 



of Peripatus capensis ....... 172 



125. Diagrammatic transverse sections through the bodies of embryos of Peri- 



patus capensis of various ages in order to illustrate the mutual relation- 

 ships of haemocoele (primary body-cavity) and coelom (secondary body- 

 cavity) ......... 173 



