Thread-cells or Urticating Organs.--Sertulariæ.--Campanulariadæ.--Hydrozoie 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. Despite the low rank they occupy in the hierarchy of animal life, the Cœlenterata, comprising the numerous families of the Jelly-fishes and Polyps, play a most important part in the household of the ocean, for the sea is frequently covered for miles and miles with their incalculable hosts, and whole archipelagos and continents are fringed with the calcareous structures they raise from the bottom of the deep. Their organisation is more simple than that of the preceding classes, for they have neither the complex intestinal tube of the polyzoa or the sea-urchins nor the jointed rays or arms of the star-fishes; their whole digestive apparatus is but a simple sac, and their instincts are reduced to the mere prehension of the food that the currents bring within reach of their tentacles, or to the retraction of these organs when exposed to a hostile attack. But, simple as they are, they have been provided by Nature with a comparatively formidable weapon in those remarkable "thread-cells," or urticating organs, which are so constantly met with in their integuments, and chiefly in their tentacles. The thread-cells are composed of a double-walled sac having its open extremity produced into a short sheath terminating in a long thread. A number of barbs or hooks are sometimes disposed spirally around the sheath, the thread itself being often delicately serrated. Under pressure or irritation the thread-cell suddenly breaks, its fluid escapes, and the delicate thread is so rapidly projected that the eye is utterly unable to follow the process. The violent protrusion of this barbed missile, along with the acrid secretion of the cell, causes many a worm or crustacean of equal or superior strength, that might have gone forth as victor from the struggle of life, to succumb to the cœlenterate, and is even in many cases exceedingly irritating to the human skin. Besides enabling its possessor to derive his subsistence from animals whose activity, as compared with his own, might be supposed to have removed them altogether out of the reach of danger, these stings serve also as admirable weapons of defence, and many a rapacious crab or annelide that would willingly have feasted upon a sea-anemone is no doubt repelled by the venomous properties of its urticating tentacles. [Illustration: Urticating Organs of Cœlenterata.