MORPHOLOGY OF THE FINS OF FISHES. Origin of the Fins of Fishes.—Origin of the Paired Fins.—Development of the Paired Fins in the Embryo.—Evidences of Palæontology.—Current Theories as to Origin of Paired Fin.—Balfour's Theory of the Lateral Fold.—Objections.—Objections to Gegenbaur's Theory.—Kerr's Theory of Modified External Gills.—Uncertain Conclusions.—Forms of the Tail in Fishes.—Homologies of the Pectoral Limb.—The Girdle in Fishes other than Dipnoans. 62 CHAPTER VI. THE ORGANS OF RESPIRATION. How Fishes Breathe.—The Gill Structures.—The Air-bladder.—Origin of the Air-bladder.—The Origin of Lungs.—The Heart of the Fish.—The Flow of Blood. 91 CHAPTER VII. THE NERVOUS SYSTEM. The Nervous System.—The Brain of the Fish.—The Pineal Organ.—The Brain of Primitive Fishes.—The Spinal Cord.—The Nerves. 109 CHAPTER VIII. THE ORGANS OF SENSE. The Organs of Smell.—The Organs of Sight.—The Organs of Hearing.—Voices of Fishes.—The Sense of Taste.—The Sense of Touch. 115 CHAPTER IX. THE ORGANS OF REPRODUCTION. The Germ-cells.—The Eggs of Fishes.—Protection of the Eggs.—Sexual Modification. 124 CHAPTER X. THE EMBRYOLOGY AND GROWTH OF FISHES. Post-embryonic Development.—General Laws of Development.—The Significance of Facts of Development.—The Development of the Bony Fishes.—The Larval Development of Fishes.—Peculiar Larval Forms.—The Development of Flounders.—Hybridism.—The Age of Fishes.—Tenacity of Life.—Effect of Temperature on Fishes.—Transportation of Fishes.—Reproduction of Lost Parts.—Monstrosities among Fishes. 131 CHAPTER XI. INSTINCTS, HABITS, AND ADAPTATIONS. The Habits of Fishes.—Irritability of Animals.—Nerve-cells and Fibers.—The Brain or Sensorium.—Reflex Action.—Instinct.—Classification of Instincts.—Variability of Instincts.—Adaptations to Environment.—Flight of Fishes.—Quiescent Fishes.—Migratory Fishes.—Anadromous Fishes.—Pugnacity of Fishes.—Fear and Anger in Fishes.—Calling the Fishes.—Sounds of Fishes.—Lurking Fishes.—The Unsymmetrical Eyes of the Flounder.—Carrying Eggs in the Mouth. 152 CHAPTER XII. ADAPTATIONS OF FISHES. Spines of the