CONTENTS xi n 



CHAPTER XV 



PAGE 



The Crayfish. Arthropoda .... 239. 



CHAPTER XVI 

 The Cockroach. Insects ..... 272 



CHAPTER XVII 

 The Nematoda. Parasitism . . 295 



CHAPTER XVIII 

 The Swan Mussel. Mollusca .... 310 



CHAPTER XIX 

 The Lancelet. Chordata . ... 326 



CHAPTER XX 

 The Dogfish ..... 340 



CHAPTER XXI 

 Cold-blooded Vertebrata . . • 374 



CHAPTER XXII 

 The Pigeon . 



396 



