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. {346}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.