The Porites and Milleporæ, which are the chief reef-building corals, cannot live at greater depths than fifteen or twenty-five fathoms: not for want of heat, for the temperature of the ocean in these latitudes does not sink to 68° Fahr. till a depth of 100 fathoms, but light and abundance of uncombined air are essential, and these decrease as the depth increases. The polypes perish if exposed directly to the sun even for a short time, so they build horizontally between these limits. The actinian polypes in the corals, which live at different depths in the crevices of the atolls, have the same general structure; their disks and tentacles are sometimes tinted with brilliant colours; some sting, others have a considerable diversity of individual character.
On the margin of the atolls, close within the line where the coral is washed by the tide, three species of Nullipores flourish; they are beautiful little plants, very common in the coral islands. One species grows in thin spreading sheets, like a lichen; the second in stony knobs as thick as a man’s finger, radiating from a common centre; and the third species, which has the colour of peach blossom, is a reticulated mass of stiff branches about the thickness of a crow’s quill. The three species either grow mixed or separately, and, although they can exist above the line of the corals, they require to be bathed the greater part of each tide: hence a layer two or three feet thick, and about twenty yards broad, formed by the growth of the Nullipores, fringes the circlet of the atolls and protects the coral below.
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.