xiv CONTENTS. 



PAGE 



V. THE PROTOCHORDATA IN THEIR RELATION TO THE 



PROBLEM OF VERTEBRATE DESCENT 242 



BALANOGLOSSUS 242 



External Features 242 



Nervous System and Gonads 244 



Metamerism 246 



Body-cavities; Proboscis-pore; Collar-pores 247 



Alimentary Canal 249 



Development; the Tornaria Larva 250 



The Larva of Asterias Vulgaris; Water-pores and Pnxoral 



Lobe 253 



Apical Plate of Tornaria 255 



Metamorphosis of Tornaria 256 



The Nemertines 256 



CEPHALODISCUS AND RHABDOPLEURA 261 



THE PR/EORAL LOBE OF ECHINODERM LARV.^E 267 



THE PR^ORAL LOBE OF THE PROTOCHORDATES 271 



Anterior and Posterior Neurenteric Canals, and the Position of 



the Mouth in the Protochordates 274 



THE PR.EOKAL LOBE IN THE CRANIATE VERTEBRATES .... 279 



THE MOUTH OF THE CRANIATE VERTEBRATES 280 



SIGNIFICANCE OF THE HYPOPHYSIS CEREBRI 283 



The Ascidian Hypophysis ... 287 



CONCLUSION 289 



NOTES 291 



REFERENCES 295 



INDEX . . 311 



