CONTENTS xiii 



Chapter 20 



THE CHORDATES 383 



174. Chordate Characteristics 383 



175. Subphylum Urochordata 384 



176. Subphylum Cephalochordata 387 



177. Subphylum Vertebrata 389 



178. The Origin of Chordates 391 



Part III. The Vertebrate Life and Organization 



Chapter 21 

 THE FROG-A REPRESENTATIVE VERTEBRATE 393 



179. Frogs and Other .Amphibians 393 



180. External Features 394 



181. Skin and Coloration 395 



182. Skeleton 397 



183. Muscular System 401 



184. Body Cavity and Mesenteries 402 



185. Digestive System 404 



186. Respiratory System 406 



187. Circulatory System 408 



188. Excretory System 411 



189. Reproductive System 412 



190. Sense Organs 414 



191. Nervous System 415 



192. Endocrine Glands 419 



193. Life Cycle 420 



Chapter 22 

 A HISTORY OF VERTEBRATES: FISHES 424 



194. Methods of Determining the History of Animals. 424 



195. Vertebrate Beginnings 427 



196. Living Jawless Vertebrates 429 



197. Jaws and Paired Appendages 431 



198. Characteristics of Cartilaginous Fishes 433 



199. Evolution of Cartilaginous Fishes 436 



200. Lungs and Swim Bladders 437 



201. Evolution of Bony Fishes 201 



