Between the plates, and in the protected crevices of the outer circle of the ring, a multitude of branching zoophytes and other productions flourish; but the Porites, Astræans, and Milleporæ seem alone able to resist the fury of the breakers, essential to the very existence of these hardy corals, which only obtain their full development when washed by a heavy sea. The outer margins of the Maldive atolls, consisting chiefly of Milleporæ and Porites, are beat by a surf so tremendous that even ships have been thrown by a single heave of the sea high and dry on the reef. The waves give innate vigour to the polypes by bringing an ever-renewed supply of food to nourish them, and oxygen to aërate their juices: besides, uncommon energy is given and maintained by the heat of a tropical sun, which gives them power to abstract enormous quantities of solid matter from the water to build their stony homes, a power that is efficient in proportion to the energy of the breakers which furnish the supply. 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.