CHAPTER XVII

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CŒLENTERATA.

POLYPS AND JELLY-FISHES.

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

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