In the Tubiporidae, or "organ-pipe corals," of which T. musica (fig. 80) is a familiar example, there is a well-developed sclerodermic corallum, with thecae, but without septa. The corallum is composed of a number of bright-red, tubular, cylindrical thecae, which are united together externally by horizontal plates or floors, which appear to be formed by periodical extensions from the mouths of the tubes. The polypes are usually bright green in colour, and possess eight tentacles each.
Fig. 80. - A, Portion of the corallum of Tubipora musica, of the natural size, showing the tubular corallites and their connecting floors. B, Polype of the same, greatly enlarged, showing the mouth and tentacles.
As shown by Prof. Perceval Wright, the tubes of Tubipora are in reality composed of fused spicules; and the polypes when alarmed retract themselves within their tubes, the upper portions of which are composed of loose fusiform spicules, and are thus capable of withdrawal into the lower dense portion of the thecae.