Hydrocorallina

Hydrocorallina, a group of Hydroids (q.v.) including those in which the skeleton is massive and calcareous, and which have generally been included among the Anthozoa. They are divided into two groups, the Millepores, and the Stylasters. The former form massive or lobed or flat expanded skeletons, through which run a double series of tubes; one set is large, and one small. The large tubes are intercepted by horizontal plates or tabulee. These tubes are occupied by large "gastrozooids" or digestive polypites, while the smaller ones which surround them are occupied by "dactylozooids." These have no mouth, and simply serve to capture food, which is digested by the gastrozooids, which supply it to the colony by means of a series of canals. The dactylozooids may be irreguleirly scattered or placed in definite circles around the gastrozooids. The Stylasters form branching plant-like colonies, on the surfaces of which open a number of apertures on raised areas. The pores are broken up by septa, which radiate from the walls towards the centre. These, however, are not true septa. In addition to the large gastrozooids, there are numerous smaller ones (dactylozooids) arranged around each of the former, the whole group forming a cyclosystem. The reproductive organs are external, and are developed in sac-like "amptdlee." This also affords clear evidence of the Hydrozoan affinities of these corals. The Millepores are generally colourless or pale yellow, but the Stylasters are a fine red tint.