VI CONTENTS. 



PAGE 



Mesenteries 103 



Splanchnocoele .......... 106 



Muscular System . . . . . . . . . 107 



Electrical Organs . . . . . . . .115 



Urogenital Organs . . . . . . . . .116 



Mesenchymatous Structures ........ 132 



Skeleton ........... 133 



Vertebral Column ........ 134 



Ribs ........... 143 



Sternum .......... 147 



Skull ........... 150 



Appendicular Skeleton ....... 167 



Organs of Circulation . . . . . . . .178 



Heart 184 



Aortic Arches . . . . . . -. . . 185 



i Arteries 188 



\ Veins . . .192 



Lymph System . . . . . . . . . 198 



The Segmentation of the Head ....... 201 



The Early History of the Ovum ....... 205 



The Origin of the Vertebrates 215 



PART II. CLASSIFICATION OF VERTEBRATES. 



Sub-phylum Vertebrata . ... . . . . . .218 



Series I. Cyclostomata . . . . . , . . .219 



Class I. Marsipobranchii . . . . . . .219 



Sub-Class I. Petromyzontes ..... 223 



Sub-Class II. Myxinoidei . . . . . 224 



Ostracodermi ... 224 



Order I. Heterostraci . . . . . .224 



Order II. Aspidocephali 225 



Order III. Antiarcha . . . . . .225 



Series II. Gnathostomata . . 225 



Grade I. Ichthyopsida . ... 226 



Class I. Pisces ......... 227 



Sub-Class I. Elasmobranchii ..... 232 



Order I. Cladoselachii ..... 237 



Order II. Ichthyotomi ..... 237 



Order III. Selachii 238 



Order IV. Holocephali 240 



Sub-Class II. Teleostomi 242 



Legion I. Ganoidea ...... 248 



Order I. Crossopterygii . 249 



Order II. Chondrostei 250 



Order III. Holostei . . o . . .251 

 Legion II. Teleostei ...... 252 



Order I. Ostariophysi 254 



