CONTENTS 



IX 



CHAPTER PAGE 



B. Organ Systems 128 



1. Nutritive System 128 



2. Respiratory and Vascular Systems .... 129 



3. Excretory System 130 



4. Reproductive System 130 



5. Development 131 



6. Nervous System 133 



C. Activities of the Workers 135 



III. Insects in General 136 



A. Polymorphism 136 



B. Economic Importance 137 



XIV. Clam 139 



A. External Structure 139 



B. Organ Systems 140 



1. Nutritive System 142 



2. Vascular System 143 



3. Respiratory System 145 



4. Excretory System 145 



5. Reproductive System 146 



6. Nervous System 147 



XV. Thf Frog and Vertebrates in General .... 149 



A. Hemichordata 150 



B. Urochordata 150 



C. Cephalcchordata 152 



D. Vertebrata 152 



1. Cyclostomata 154 



II. Elasmobranchii 155 



III. Pisces 155 



IV. Amphibia 157 



V. Reptilia 157 



VI. Aves 159 



VII. Mammalia 159 



The Frog 



A. External Structure 165 



B. The Rody Tissues 167 



1. Epithelial Tissue 167 



2. Muscular Tissue 169 



3. Supporting and Connective Tissues .... 172 



4. Skeleton 175 



C. Internal Structure 181 



D. Organ Systems 184 



1. Nutritive System 184 



2. Respiratory System 189 



3. Vascular, or Circulatory, System .... 192 



