The lagoon in the centre of these islands is supplied with water from the exterior by openings in the lee side of the ring, but as the water has been deprived of the greater part of its nutritious particles and inorganic matter by the corals on the outside, the hardier kinds are no longer produced, and species of more delicate forms take their place. The depth of the lagoon varies in different atolls from fifty to twenty fathoms or less, the bottom being partly detritus, partly live coral. In these calm and limpid waters the corals are of the most varied and delicate structure, of the most charming and dazzling hues. When the shades of evening come on, the lagoon shines like the milky way with myriads of brilliant sparks. The microscopic medusæ and crustaceans invisible by day form the beauty of the night, and the sea-feather, vermilion in daylight, now waves with green phosphorescent light. This gorgeous character of the sea bed is not peculiar to the lagoons of the atolls; it prevails in shallow water throughout the whole coral-bearing regions of the Pacific and Indian Oceans. Encircling reefs differ in no respect from the atoll ring, except in having islands in their lagoons, surrounded also by coral reefs. Barrier reefs are of the same structure as the atoll rings, from which they only differ in their position with regard to the land. They form extensive lines along the coasts, from which they are separated by a channel of the sea of variable depth and breadth, sometimes large enough for ships to pass. A very long one runs parallel to the west coast of New Caledonia, and stretches for 120 miles beyond the extremities of the island. But a barrier reef off the northeastern coast of the Australian continent is the grandest coral formation existing. Rising at once from an unfathomable depth of the ocean, it extends for a thousand miles along the coast with a breadth varying from 200 yards to a mile, and at an average distance of from 20 to 60 or 70 miles from the coast, the depth of the channel being from 10 to 60 fathoms. The pulse of the ocean, transcendently sublime, beats perpetually in peals of thunder along that stupendous reef, the fabric of almost microscopic beings. SECTION V. ANNULOSA, OR WORMS. THE Annulosa, which are the lowest grade of articulated animals, consist of four distinct orders: the Entozoa, which are muscle and intestine parasites; the Turbellariæ, fresh and salt-water animals covered with cilia; the Annelida, or Worms; and the Rotifers, or Wheel animalcules. _Entozoa._