The changeability of colour may remind us of the " calices versicolores " which Hadrian sent to Servianus.
Larger star-shaped cavities, called calices, in which the zooids are lodged, and very numerous smaller round or polygonal apertures, which in life contain as many short unbranched A FIG.
B A, Portion of the surface of a colony of Heliopora coerulea magnified, showing two calices and the surrounding coenenchymal tubes.
The walls of the calices and coenenchymal tubes are formed of flat plates of calcite, which are so disposed that the walls of one tube enter into the composition of the walls of adjacent tubes, and the walls of the calices are formed by the walls of adjacent coenenchymal tubes.
The cavities both of the calices and coenenchymal tubes of Heliopora are closed below by horizontal partitions or tabulae, hence the genus was formerly included in the group Tabulata, and was supposed to belong to the madreporarian corals, both because of its lamellar skeleton, which resembles that of a Ma.drepore, and because each calicle has from twelve to fifteen radial partitions or septa projecting into its cavity.