X CONTENTS 



74. Dugesia: Sensation and Movement 207 



75. Dugesia: Water Balance and Excretion 208 



76. Dugesia: Reproduction 209 



77. Dugesia: Regeneration and Polarity 211 



78. Class Turbellaria 212 



79. Class Trematoda 213 



80. Class Cestoda 216 



Chapter 12 



THE PHYLA ASCHELAHNTHES AND NEMERTEA 220 



81. Classification of the Aschelminthes 220 



82. Class Rotifera 222 



83. Philodina 222 



84. Reproduction in Rotifers 225 



85. Cell Constancy 225 



86. Senescence 226 



87. Resistance to Desiccation 226 



88. Class Nematoda 227 



89. The \'inegar Eel. Turbatrix aceti 228 



90. The Pig Roundworm, Ascaris lumhricoides .... 229 



91. Molting 231 



92. Parasitism 231 



93. Class Gastrotricha 231 



94. Class Kinorhyncha 231 



95. Class Gordiacea 232 



96. Class Acanthocephala 232 



97. Phylum Nemertea 232 



Chapter 13 



INTRODUCTION TO THE HIGHER IN\'ERTEBRATES .... 236 



98. Evolutionary Relationships of the Sponges .... 236 



99. Evolutionary Relationships of the Coelenterates . 237 



100. The Evolution of Three Germ Layers 237 



101. The Evolution of the Coelom 238 



102. Spiral Cleavage and Its Evolutionary Importance 239 



103. The Schizocoelomata and Enterocoelomata .... 241 



Chapter 14 



THE PHYLUM MOLLUSCA 244 



104. General Features of the Molluscs 244 



105. Class Amphineura 246 



106. Class Gastropoda: General Features 247 



