CONTENTS 



17 THE VITAL FUNCTIONS AND ORGAN SYSTEMS OF INVERTEBRATES 510 



SYSTEMS RELATED TO METABOLISM 5 1 1 



Digestive Systems 511 



Circulatorv Systems 513 



Respiratory Systems 516 



Excretory Systems 518 

 SYSTEMS RELATED TO RESPONSIVENESS AND INTEGRATION 521 



Sensory-neuro-motor Systems 522 



Endocrine Mechanisms 525 

 SUPPORTING mechanisms: THE SKELETON 527 

 REPRODUCTIVE SYSTEMS AND THEIR FUNCTIONS 529 

 SUMMARY 536 



18 THE PHYLA HEMICHORDATA AND CHORDATA 538 



THE PHYLUM HEMICHORDATA 542 

 The Balanoglossida 542 

 Tlie Pterobranchia 543 

 Relationships of the Hemichordata 543 

 THE PHYLUM CHORDATA 545 

 THE ACRANIATA 545 



The Urochordata 545 

 The Cephalochordata 550 

 THE CRANIATA OR VERTEBRATA 553 

 The Pisces or Fishes 554 



The Tetrapoda or Four-Footed Vertebrates 560 

 SUMMARY 591 



19 ECOLOGY: ENVIRONMENTAL RELATIONSHIPS 594 



THE PHYSICOCHEMICAL ENVIRONMENT 596 



Temperature 596 



Oxygen 599 



Water and Salts 601 



General Considerations 604 

 THE BIOTIC ENVIRONMENT 605 



Food Relations 605 



Animal Populations, Competition, and the "Balance of Nature" 609 



Ecological Succession 614 



20 THE EVOLUTION OF ANIMAL LIFE 616 



THE ORIGIN OF LIFE 618 

 ORGANIC EVOLUTION 619 



EVIDENCE FOR THE FACT OF EVOLUTION 619 

 EVIDENCE FROM DISTRIBUTION 619 

 Geologic Distribution 620 



