Sub-Order 2. Axifera.
The axis in this sub-order may be horny, or horny with a core of calcium carbonate, or composed of horn impregnated with calcium carbonate, or of nodes of horn alternating with internodes of calcium carbonate. It may be distinguished from the axis of the Pseudaxonia by the fact that in no case have definite spicules been observed to take part in its formation. It has been suggested that as the Axifera represent a line of descent distinct from that of the Pseudaxonia they should be placed in a separate order. Apart from the character of the axis, however, the two sub-orders show so many affinities in their general anatomy that it is better to regard the two lines of descent as united within the Gorgonacean limit. It is very improbable that the two groups sprang independently from a stoloniferous ancestor.
Fam. 1. Isidae.—This family includes all those Axifera in which the axis is composed of alternate nodes of horn and internodes of calcareous substance.
There can be little doubt of the close affinities of many of the genera of this family with the Melitodidae among the Pseudaxonia. In both the coenenchym is thin and the coelenteric cavities short. No important differences have been observed between the structure of the zooids of the two families, and now that we know that the "nutritive canals" of Melitodes do not perforate the nodes there is no important difference left between the coenosarcal canal systems. The structure and method of calcification of the internodes of the two families are very similar. The main difference between them is that the nodes of the Isidae are purely horny, whereas in the Melitodidae the horny substance of the nodes contains calcareous spicules.
The principal genera are:—Isis, Linnaeus; Pacific Ocean. This genus forms substantial fan-shaped colonies with, relatively, a thick coenenchym, short stout internodes and black horny nodes. Mopsea, Lamouroux; Coast of Australia. The verrucae are club-shaped and are arranged in spiral rows round the stem. Acanella, Gray; principally found in deep water in the Atlantic Ocean but also in the Pacific. The internodes are long and the branches arise from the nodes. Most of the species occur in deep water, some in very deep water (A. simplex, 1600 to 1700 fathoms). In this and the following genera the coenenchym is thin and the zooids imperfectly or not retractile. Ceratoisis, Wright; Atlantic Ocean, extending from shallow to deep water. The branches arise from the nodes. Chelidonisis, Studer; deep water off the Azores. Isidella, Gray; Mediterranean Sea. Bathygorgia, Wright; off Yokohama, 2300 fathoms. This genus is unbranched, with very long internodes and short nodes. The zooids are arranged on one side only of the stem.
Fam. 2. Primnoidae.—This is a well-marked family. The axis of the colonies is horny and calcareous. The coenenchym and the non-retractile zooids are protected by scale-like spicules, which usually overlap and form a complete armour for the protection of the soft parts. On the aboral side of the base of each tentacle there is a specialised scale, and these fit together, when the tentacles are folded over the peristome, to form an operculum.
The principal genera are:—Primnoa, Lamouroux; Atlantic Ocean, occurring also in the Norwegian fjords. This genus is usually found in moderately deep water, 100 to 500 fathoms. Primnoella, Gray. This genus seems to be confined to the temperate seas of the southern hemisphere. It is unbranched. The zooids are arranged in whorls round the long whip-like stem. Plumarella, Gray; southern hemisphere, in moderately deep water. This is branched pinnately in one plane. The zooids are small and arise at considerable intervals alternately on the sides of the branches. Stenella, Gray; widely distributed in deep water. The zooids are large and are arranged in whorls of three situated at considerable distances apart. Stachyodes, W. and S.; Fiji, Kermadecs, Azores, in deep water. Colony feebly branched. Zooids in regular whorls of five. Other genera belonging to this group of Primnoidae are Thouarella, Gray, and Amphilaphis, Antarctic seas.
The following genera are placed in separate sub-families:—Callozostron, Wright; Antarctic Sea, 1670 fathoms. The axis is procumbent and the zooids are thickly set in rows on its upper surface. The zooids are protected by large imbricate scales, of which those of the last row are continued into long spine-like processes. Calyptrophora, Gray; Pacific Ocean, in deep water. The base of the zooids is protected by two remarkably large scales. Primnoides, W. and S.; Southern Ocean. The opercular scales are not distinctly differentiated and the calyx is therefore imperfectly protected.
Fam. 3. Chrysogorgiidae.[[384]]—The axis in this family is composed of a horny fibrous substance with interstratified calcareous particles, and it springs from a calcareous plate, which sometimes gives off root-like processes. It may be unbranched or branched in such a way that the branches of the second, third, and subsequent orders assume in turn the direction of the base of the main axis. The axis is frequently of a metallic iridescent appearance. The zooids usually arise in a single straight or spiral row on the branches, and are not retractile. The coenenchym is thin. The spicules vary considerably, but in a very large proportion of the species they are thin, oval, or hour-glass plates (Fig. 149, 10, p. [336]).
By some authors this family is considered to be the simplest and most primitive of the Axifera; but the delicate character of the axis of the main stem and branches, the thinness of the coenenchym, the position of the zooids on one side of the branches only, and the tenuity of the calcareous spicules may be all accounted not as primitive characters, but as special adaptations to the life in the slow uniform currents of deep water.