TABLE OF CONTENTS. IX 



PAGE 



Principal and Accessory Tissues 118 



Effect of Use and Disuse 119 



Change of Function of Organs 119 



Homology and Analogy 120 



Systems of Organs 120 



Vegetal and Animal Organs 121 



Vegetal Organs 122 



A. Organs of Assimilation 122 



Assimilation Defined 122 



Assimilation in Animals 122 



Different Organs of Assimilation 122 



I. The Digestive Tract. 



Archenteron, or Primitive Digestive Tract .... 124 



Stomodceum and Proctodceum 125 



Various Divisions and Appendages of the Digestive 



Tract 126 



II. Respiratory Organs 128 



Sources of the Oxygen Used in Breathing .... 128 

 Distinctions between the Respiratory Systems of 



Vertebrates and Invertebrates 130 



III. Circulatory Apparatus 131 



Ccelom 132 



Heart, Arteries, Veins, Capillaries 132 



The Higher Development of the Heart ..... 132 



Blood-vessels 133 



Correlation of Respiratory Organs and Blood-system 133 



Arterial and Venous Blood 135 



Closed and Lacunar Blood-vascular Systems . . . 135 



Example of Lacunar Blood-vascular System . . . 136 



Lymph-vessels 136 



Cold- and Warm-blooded Animals 137 



iv. Excretory Organs 138 



Nature of the Organs of Excretion 138 



Nephridia 140 



B. Sexual Organs 140 



Sexual Glands and Outlets 140 



Germinal Epithelium and Germinal Glands .... 140 



Gonochorism and Hermaphroditism 141 



Occurrence of Hermaphroditism 142 



True Hermaphroditism 143 



Outlets 143 



Accessory Sexual Apparatus 144 



Animal Organs 144 



I. Organs of Locomotion 144 



Voluntary Locomotion 144 



Locomotion among the Lower Animals 145 



