A remarkable genus is Coelogorgia. Here we find a branching colony arising from a basal stolon, and the axis of the main stem and of each branch consists of a single very much elongated zooid bearing on its thickened walls the branches of the next series and other zooids. It is true that in this genus there is very little fusion of neighbouring zooids, and the amount of true coenenchym is so small that it can hardly be said to exist at all. Bourne[[373]] has united this genus with Telesto into a family Asiphonacea, which he joins with the Pennatulida in the order Stelechotokea; but their affinities seem to be closer with the Alcyonacea than with the Pennatulacea, from which they differ in many important characters.
Fig. 154.—Alcyonium digitatum, a single-lobed specimen, with some of the zooids expanded.
The genus Alcyonium not only contains the commonest British Alcyonarian (A. digitatum), but it is one of the most widely distributed genera of all Alcyonaria that occur in shallow water.
The genera Sarcophytum and Lobophytum occur in shallow water in the tropics of the old world. The former frequently consists of huge toad-stool shaped masses, soft and spongy in consistency, of a green, brown, or yellow colour. On some reefs the colonies of Sarcophytum form a very conspicuous feature, and from their very slimy, slippery surface, add to the minor dangers of wading in these regions. Both genera are dimorphic. Some species of the genus Sclerophytum,[[374]] which occur in the Indian Ocean, are so hard and brittle that they might readily be mistaken for a Zoantharian coral. This character is due to the enormous number of tightly packed spicules borne by the coenenchym. Some of these spicules in S. querciforme are 7 mm. × 1.7 mm.; the largest, though not the longest (vide p. [335]) of any spicules occurring in the order.
Another very important genus occurring on coral reefs, and of very wide distribution, is Spongodes. This genus forms bushy and rather brittle colonies of an endless variety of beautiful shapes and colours. Arising from the neck of each anthocodia there are one or two long, sharp, projecting spicules, which give the surface a very spiny or prickly character.
The genera Siphonogorgia and Chironephthya form large brittle, branching colonies which might readily be mistaken for Gorgonians. The strength of the branches, however, is mainly due to the large, densely packed, spindle-shaped spicules at the surface of the coenenchym, the long coelenteric cavities of the zooids penetrating the axis of both stem and branches. Siphonogorgia is usually uniformly red or yellow in colour. Chironephthya, on the other hand, exhibits a great variety of colour in specimens from the same reef, and indeed in different branches of the same colony.
Fam. 1. Xeniidae.—Alcyonacea with non-retractile zooids. Spicules very small discs, usually containing a relatively small proportion of lime.
Xenia, Savigny; Indian Ocean and Torres Straits. Heteroxenia, Kölliker; Red Sea, Cape of Good Hope, and Torres Straits.
Fam. 2. Telestidae.—Colonies arising from an encrusting membranous or branching stolon. The erect stem and branches are formed by the body-walls of two or three zooids only, from which secondary zooids and branches of the next order arise.