Thread-cells or Urticating Organs.--Sertulariæ.--Campanulariadæ.-- Hydrozoic Acalephæ.--Medusidæ.--Lucernariadæ.--Calycophoridæ.-- The Velella.--The Portuguese Man-of-war.--Anecdote of a Prussian Sailor.--Alternating Fixed and Free-swimming Generations of Hydrozoa.--Actinozoa.--Ctenophora.--Their Beautiful Construction. --Sea-anemones.--Dead Man's Toes.--Sea-pens.--Sea-rods.--Red Coral.--Coral Fishery.--Isis hippuris.--Tropical Lithophytes.-- History of the Coral Islands.--Darwin's Theory of their Formation.--The progress of their Growth above the level of the Sea 345 CHAPTER XVIII. PROTOZOA. The Foraminifera.--The Amœbæ.--Their Wonderful Simplicity of Structure.--The Polycystina.--Marine Infusoria.--Sponges. --Their Pores.--Fibres and Spiculæ.--The Common Sponge of Commerce 378 CHAPTER XIX. MARINE PLANTS. The Algæ.--Zostera marina.--The Ulvæ and Enteromorphæ.--The Fuci. --The Laminariæ.--Macrocystis pyrifera.--Description of the Submarine Thickets at Tierra del Fuego.--Nereocystis lutkeana. --The Sargasso Sea.--The Gathering of edible Birds'-nests in the marine Caves of Java.--Agar-Agar.--The Florideæ.--The Diatomaceæ.--Their importance in the economy of the Seas 390 CHAPTER XX. THE GEOGRAPHICAL DISTRIBUTION OF MARINE LIFE. The Dependence of all created Beings upon Space and Time.--The Influences which regulate the Distribution of Marine Life.--The four Bathymetrical Zones of Marine Life on the British Coasts, according to the late Professor Edward Forbes of Edinburgh. --Abyssal Animals.--_Bathybius Haeckelii._--Deep-Sea Sponges and Shell-Fish.--Vivid Phosphorescence of Deep-Sea Animals.--Deep-Sea Shark Fishery--The "Challenger." 405 CHAPTER XXI. THE PHOSPHORESCENCE OF THE SEA. Its Causes.--Noctiluca miliaris.--Phosphorescent Annelides and Beroës.--Intense Phosphorescence of the Pyrosoma atlantica. --Luminous Pholades.--The luminous Shark.--Phosphorescent Algæ. --Citations from Byron, Coleridge, Crabbe, and Scott. 423 CHAPTER XXII. THE PRIMITIVE OCEAN. The Giant-Book of the Earth-rind.--The Sea of Fire.--Formation of a solid Earth-crust by cooling.--The Primitive Waters.--First awakening of Life in the Bosom of the Ocean.--The Reign of the Saurians.--The future Ocean. 433 PART III. THE PROGRESS OF MARITIME DISCOVERY. CHAPTER XXIII.