CONTENTS. 



PAGE 



INTRODUCTION . . 



HISTORICAL SKETCH v 



LIST OF AUTHOR'S PAPERS ... xi 



CHAPTER I. xiv 



OUTLINE OF THE ARACHNID THEORY 1-26 



I. Its scope and relation to other theories, i. II. Nature of the evidence to be pre- 

 sented, 3; A. cephalogenesis in arthropods, 3; B. embryology, 4; C. arachnid 

 cephalogenesis prophetic of the vertebrate head, 5; D. paleontology. III. The 

 process of cephalization in the arthropods, 7 ; A. The grouping and the increase in 

 number of metamers, 7; B. origin of the linear arrangement of unlike cephalic 

 functions, 8. IV. The subdivisions of the incipient vertebrate head in arachnids, 

 ii ; mesoderm, 12. i. The procephalon, 12; insects, 13; arachnids, 13; sense 

 organs, 13; olfactory lobes, hemispheres and optic ganglia, 13; rostrum, 14; exter- 

 nal boundaries of the procephalon in the adult, 14. 2, 3. The dicephalon and the 

 mesocephalon, 15; endocranium, 16; oral arches, 16; taste buds, slime buds and 

 cranial ganglia, 17; segmental sense organs, 17; the diencephalon and the mesen- 

 cephalon, 18; the suprastomodaeal commissure and the cerebellum, 19. 4. The 

 metencephalon, or vagus region, 19; vagus appendages, 19; vagus neuromeres, 19; 

 vagus nerves, 20. 5. The branchiocephalon, 20; mesoderm, 20; neuromeres, 21; 

 nerves, 21; the endocranium, 21; the mesoderm, 22; comparison, 23; the vascular 

 area and concrescence, 24; the new mouth, cephalic navel or haemostoma, 25; the 

 closure of the old mouth or neostoma, 26; conclusion, 26. 



CHAPTER II. 



OUTLINE OF THE ARACHNID THEORY; CONTINUED 27-40 



I. Comparison of adult arthropods with adult vertebrates, 27. I. Orientation of 

 neural and haemal surfaces, 27. II. Comparison of adult arthropods and verte- 

 brates, 29; bunodes, 29. III. Comparison of arthropod and vertebrate embryos, 

 33; form controlling factors in the early stages, 34; the gustrula, ccelenterate, or 

 trochosphere stage, 34; transition from radiate to bilateral symmetry, 35; telopore, 

 35; germ wall, 35; concrescence of the germ wall, 35; the nervous system, 38; the 

 primary sense organs, 38; cornua, 38; vertebrate stages, 38; the auditory pit, 39; 

 the heart, 39; cornua, 40; the oral arches and the haemostoma, 40; cranial flex- 

 ure, 40. 



CHAPTER III. 



EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS 4i~52 



I. Meaning of the term brain, 41. II. The sternodceal nerves, 42. III. The 

 framework of the nervous system, 43. IV. The differentiation of the peripheral 

 nerves, 45; factors that modify the arrangement of peripheral nerves, 46; segrega- 



xi 



