It is found at the present day in many localities in the warm shallow waters of the tropical Pacific and Indian Oceans. It usually flourishes on the inside of the reef, and may form masses of stone five or six feet in diameter. The coral may easily be recognised, as it is the only one that exhibits a blue colour. This colour usually penetrates the whole skeleton, but in some forms is absent from the superficial layers.
The skeleton consists of a number of parallel tubes with imperforate walls, which are fused together in honey-comb fashion. On making a vertical section through a branch of the coral it is found that the tubes are divided into a series of chambers by transverse partitions or "tabulae." The soft living tissues of the coral, the zooids and coenosarc, are confined to the terminal chambers, all the lower parts being simply dead calcareous skeleton supporting the living superficial layer. Among the parallel tubes there may be found a number of larger chambers that seem to have been formed by the destruction of the adjacent walls of groups of about nineteen tubes. These chambers are provided with a variable number of pseudo-septa, and have a remarkable resemblance to the thecae of some Zoantharian corals. That Heliopora is not a Zoantharian coral was first definitely proved by Moseley, who showed that each of these larger chambers contains an Alcyonarian zooid with eight pinnate tentacles and eight mesenteries. The zooids arise from a sheet of coenosarc that covers the whole of the living branches of the coral mass, and this sheet of coenosarc bears a plexus of canals communicating on the one hand with the zooids, and on the other with a series of blind sacs, each of which occupies the cavity of one of the skeletal tubes as far down as the first tabula. The zooids of Heliopora are very rarely expanded during the day-time, and it has been found very difficult to get them to expand in an aquarium. The coral, however, is frequently infested with a tubicolous worm allied to the genus Leucodora, which freely expands and projects from the surface. So constant and so numerous are these worms in some localities that it has actually been suggested that Heliopora should be regarded as a Polychaete worm and not as an Alcyonarian. According to Mr. Stanley Gardiner, however, these worms do not occur in association with the Heliopora found on the reefs of the Maldive Archipelago.
There is very strong reason to believe that certain fossil corals were closely related to Heliopora; that Heliopora is in fact the solitary survivor of a group of Alcyonarian corals that in past times was well represented on the reefs, both in numbers and in species. The evidence is not so convincing that other fossil corals are closely related to Heliopora, and their true zoological position may remain a matter for surmise. The order may be classified as follows:—
Fam. 1. Heliolitidae.[[371]]—Coenothecalia with regular, well-developed septa, generally twelve in number, in each calicle.
Heliolites, Dana, Silurian and Devonian. Cosmiolithus, Lindström, Upper Silurian. Proheliolites, Klaer, Lower Silurian. Plasmopora, Edwards and Haime, Upper Silurian. Propora, E. and H., Upper Silurian. Camptolithus, Lindström, Upper Silurian. Diploëpora, Quenst, Upper Silurian. Pycnolithus, Lindström, Upper Silurian.
Fam. 2. Helioporidae.[[372]]—Coenothecalia with small irregularly arranged coenosarcal caeca, and a variable number of septa or septal ridges. Heliopora, de Blainville, recent, Eocene and Upper Cretaceous. Polytremacis, d'Orbigny, Eocene and Upper Cretaceous. Octotremacis, Gregory, Miocene.
The family Coccoseridae is regarded by Lindström as a sub-family of the Heliolitidae, and the families Thecidae and Chaetetidae are probably closely related to the Helioporidae.
Order III. Alcyonacea.
This order contains a large number of genera of great variety of form. The only characters which unite the different genera are that the body-walls of some groups of zooids, or of all the zooids, are fused together to form a common coenenchym penetrated by the coenosarcal canals, and that the spicules do not fuse to form a solid calcareous, or horny and calcareous, axial skeletal support.
The affinities with the order Stolonifera are clearly seen in the genera Xenia and Telesto. Some species of Xenia form flattened or domed colonies attached to stones or corals, with non-retractile anthocodiae and body-walls united for only a short distance at the base. Young Xenia colonies are in fact Stolonifera in all essential characters. In Telesto prolifera we find a network of stolons encrusting coral branches and other objects after the manner of the stolons of many species of Clavularia, although the zooids do not arise from these stolons singly, but in groups, with their body-walls fused together for a certain distance. In Telesto rubra the spicules of the body-walls are fused together to form a series of perforated tubes very similar in some respects to the tubes of Tubipora.