The Alcyonarians of this group possess a well-developed sclerodermic corallum, composed of tabulate tubes of two sizes, the larger ones being furnished with rudimentary septal laminae.

The family Helioporidae has been recently founded by Mr Mose-ley for the reception of the living Heliopora caerulea (fig. 84), and of a number of extinct corals previously placed in the "Tabulate" section of the Zoantharia sclerodermata. In Heliopora the corallum is composite and sclerodermic, and composed of corallites united by what has usually been regarded as a "coenenchyma." The corallites are tubular, crossed by welldeveloped tabulae, and having their walls folded in such a manner as to give rise to a variable number (generally twelve) of septal laminae. The coenenchyma, so called, is composed of slender tubes, of smaller size than the true corallites, packed closely side by side, crossed, like the corallites, by regular transverse tabulae, but destitute of septa. The soft parts occupy only the parts of the corallum above the uppermost tabulae, and therefore only a surface-layer of the colony is actually alive. The polypes are completely retractile, with eight pinnately-fringed tentacles, and eight mesenteries. The mesenteries, however, have no correspondence with the septa, which are twelve in number as a rule. The septa are thus seen to be pseudo-septa, and they cannot be regarded as being homologous with the septa of the Zoantharia sclerodermata. The so-called coenenchymal tubes are occupied by sacs lined by the endoderm, which are closed externally, but communicate freely with the body-cavities of the polypes by means of transverse canals; and Mr Moseley suggests, with great probability, that these are really of the nature of rudimentary sexless polypes.

Family V Helioporidae 106Family V Helioporidae 107Fig. 84.   A, Colony of Heliopora caertdea, of the natural size. B, Portion of the surface of the same, enlarged, showing the apertures of the larger and smaller zooids. C, Vertical section of a few of the tubes of the same, enlarged, showing the tabulae. (After Dana.)

Fig. 84. - A, Colony of Heliopora caertdea, of the natural size. B, Portion of the surface of the same, enlarged, showing the apertures of the larger and smaller zooids. C, Vertical section of a few of the tubes of the same, enlarged, showing the tabulae. (After Dana.)

Now that the fact is established that the living Heliopora is a true Alcyonarian, it is necessary to remove to this order a number of well-known fossil corals, principally Palaeozoic, of which Heliolites may be taken as the type, and which were formerly regarded as belonging to the "Tabulate" section of the Zoantharia sclerodermata. In Heliolites (fig. 85), there is a well - developed sclerodermic corallum, with comparatively large-sized, tubular, regularly tabulate corallites, usually possessing distinct but rudimentary septa, intermingled with a copious coenenchyma formed of tabulate geometric tubuli, much smaller than the corallites, and destitute of septa. With Heliolites must be placed the equally extinct Plasmopora, Propora, Polytremacis, etc.

Family V Helioporidae 109Family V Helioporidae 110Fig. 85.   A, Small colony of Heliolites megastoma, of the natural size. B, Small portion of the surface of the same, magnified, showing the calices (a) and the mouths of the coenenchymal tubes (b). C, Vertical section of the same, enlarged, showing the tabulate corallites (a), and the tabulate tubes of the coenenchyma (b). (Original.)

Fig. 85. - A, Small colony of Heliolites megastoma, of the natural size. B, Small portion of the surface of the same, magnified, showing the calices (a) and the mouths of the coenenchymal tubes (b). C, Vertical section of the same, enlarged, showing the tabulate corallites (a), and the tabulate tubes of the coenenchyma (b). (Original.)