| |||||||||
| |||||||||
The description of the internal arrangement of parts in the Actinia applies in every particular to these corals, with the exception of the hard deposit in the lower part of the body. As in all the Polyps, radiating partitions divide the main cavity of the body into distinct separate chambers, and the tentacles increasing by multiples of six, numbering six in the first set, six in the second, and twelve in the third, are hollow, and open into the chambers. But the feature which distinguishes them from the soft Actiniæ, and unites them with the corals, requires a somewhat more accurate description. In each individual, a hard deposit is formed ([Fig. 20]), beginning at the base of every chamber, and rising from its floor to about one fifth the height of the animal at its greatest extension. This lime deposit does not, however, fill the chamber for its whole width, but rises as a thin wall in its centre. (See [Figs. 13], [17].) Thus between all the soft partitions, in the middle of the chambers which separate them, low limestone walls are gradually built up, uniting in a solid column in the centre. These walls run parallel with the soft partitions, although they do not rise to the same height, and they form the radiating lines like stiff lamellæ, so conspicuous when all the soft parts of the body are drawn in. The mouth of the Astrangia is oval, and the partitions spread in a fan-shaped way, being somewhat shorter at one side of the animal than on the other. The partitions extend beyond the solid wall which unites them at the periphery, in consequence of which, this wall is marked by faint vertical ribs.
HALCYONOIDS.
Halcyonium. (Halcyonium carneum Ag.)