Some corals increase both by budding and division, but by far the greater number grow in size by budding, as the Astræa, which constitutes a portion of the reef-building corals of the tropical seas. They form groups, in which the whole of the polypes, except their starry summits, are soldered or pasted together by a living viscous substance, consolidated by carbonate of lime, abstracted from the sea-water, so that the resulting coral frequently becomes a rounded mass, the surface of which is more or less covered with stars, which may be circular or angular, large or small, deeply set or prominent, according to the genera or species, both of which are exceedingly numerous. In fact the forms produced vary according as the buds spring from the base of the polypes, from the sides of the cylindrical body, from the summit or disk, from the limits of these three parts, or from the whole animal. In all these varieties the buds are the result of a superabundance of vital activity in the part. When the buds proceed from the sides of the polypes the corals are rounded masses; but when they spring from the disk or cups of the star, the consequence is the death of the parent polypes, and the development of a new layer of living individuals above the dead ones. No part of the new polypes is seen except their stars, their bodies being enclosed in the common tissue. As this process may be continued indefinitely, the coral may increase to any size; but the size becomes still greater when successive buds are formed over every part of the polypes, and when all the successive generations are soldered together by the common tissue. In every case the polypes are alive only on the surface where they have free access to light, heat, and air, which is furnished by the sea-water in which they live.[[31]]
In the reef-building corals the living viscous substance that covers the surface and connects the polypes into a mass, is in process of time so completely consolidated by abstracting the small quantity of carbonate of lime that the sea-water contains, that little if any animal matter remains; and as this process is continually repeated, one generation of polypes perishes after another, the inert matter increases indefinitely, and the surface at which the consolidation is actually going on is the only part that is alive.
The surfaces of the dense convex masses of many of these Astræan corals are entirely covered with deep hexagonal stars, whose rays extend upwards all round, and end in narrow, sharp, and elevated lines formed by the junction of the rays of the adjacent stars; in other species the rays are often crowded together, and the columella only shows a few points in the deep hollows. Through these deep cups the polypes protrude their circular disks and tentacles in quest of food, the nutritious products of which maintain the polypes as well as the general living fabric which unites them, and the refuse is ejected from their mouths; for each polype has an independent life of its own besides the incidental life that it possesses as part of a compound being. In many of the corals the polypes show great sensibility, shrinking into their cells on the slightest touch, yet no nervous system has been discovered.
The variety of compact and branching corals far exceeds description: 120 species are inhabitants of the Red Sea alone, and an enormous area of the tropical Pacific is everywhere crowded with the stupendous works of these minute agents, destined to change the present geological features of the globe, as their predecessors have done in the remote ages of its existence.
Four distinctly different formations are due to the coral-building polypes in the Pacific and Indian Oceans, namely, lagoon islands or atolls, encircling reefs, barrier reefs, and coral fringes, all nearly confined to the torrid zone.
An atoll is a ring or chaplet of coral, enclosing a lagoon or portion of the ocean in its centre. The average breadth of that part of the ring which rises above the surface of the sea is about a quarter of a mile, often less, and it is seldom more than from six to ten or twelve feet above the waves: hence the lagoon islands are not visible even at a very small distance, unless when they are covered by the cocoa-nut palm or the pandanus, which is frequently the case. On the outside, the ring or circlet shelves down for a distance of one or two hundred yards from its edge, so that the sea gradually deepens to about twenty-five fathoms, beyond which the sides of the ring plunge at once into the unfathomable depths of the ocean with a more rapid descent than the cone of any volcano. Even at the small distance of some hundred yards no bottom has been reached with a sounding line a mile and a half long. All the coral on the exterior of the ring, to a moderate depth below the surface of the water, is alive; all above it is dead, being the detritus of the living part washed up by the surf, which is so heavy on the windward side of the tropical islands of the Pacific and Indian Oceans, that it is often heard miles off, and is frequently the first warning to seamen of their approach to an atoll.
On the inside, these coral rings shelve down into the clear calm water of the lagoon by a succession of ledges of living corals, but of much more varied and delicate kinds than those on the exterior wall and foundation of the atoll. The corals known as Porites are the chief agents in building the exterior face of the ring: they form great rounded irregular masses, like the Astræa, but much larger, being many feet in thickness; and as the polypes are only alive on the surface, numberless generations must have lived and died before they could have arrived at that size. The rays of the stars are toothed at the edges, so that they present rows of little points; in some species the rays are almost invisibly slender, the interstitial matter is full of pores, and the polypes have twelve tentacles.
The Millepora complanata or palmipora is very commonly associated with the Porites; it is the largest coral known. It grows in thick vertical plates, intersecting each other at various angles, and forms an exceedingly strong honey-combed mass, generally affecting a circular form, the marginal plates alone being alive. Instead of stars, the polypes live in simple pores: myriads of these small cylindrical pores penetrate the surface of the plates perpendicular to their axes; sometimes they are so minute as to be scarcely visible.
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.