Fig. 149.—Spicules of Alcyonaria. 1, Club of Juncella; 2, warted cross of Plexaurella; 3, torch of Eunicella; 4, needle of Renilla; 5, warted spindle of Gorgonella; 6, spicule of Pennatula; 7, foliate club of Eunicea; 8, irregular spicule of Paramuricea; 9, scale of Primnoa; 10, spicules of Trichogorgia. (5 and 10 original, the remainder after Kölliker.)
In Paragorgia and some other closely related genera the spicules of the axis of the colony also become tightly wedged together, but the core thus formed is far more porous and brittle than it is in the Coralliidae. In Tubipora (the organ-pipe coral) and in Telesto rubra the spicules of the body-walls of the zooids fuse to form perforated calcareous tubes. In some species of Sclerophytum the large spicules of the coenenchym become so closely packed that they form dense stony masses, almost as hard as a Perforate Madreporarian coral. The horny substance, allied chemically to keratin, plays an important part in the building up of skeletal structures in many Alcyonaria. In Clavularia viridis and in Stereosoma a change in the chemical character of the mesogloea of the body-walls of the polyps leads to the formation of a horny tube, which in the former case is built up of interlacing fibres, and in the latter is formed as a homogeneous sheath. In many of the Alcyonacea which have a compact axial skeleton the spicules are cemented together by a horny matrix.
In the Gorgonellidae and some others the hard axis is formed of a horny substance impregnated with a crystalline form of calcium carbonate; but in the Gorgoniidae, many of the Pennatulacea and some other genera very little or no carbonate of lime is found in the horny axis.
The skeleton of the genus Heliopora differs from that of all the other Alcyonaria in its development, structure, and form. In the words of Dr. G. C. Bourne,[[365]] "the calcareous skeleton of Heliopora is not formed from spicules developed within cells but is a crystalline structure formed by crystallisation of carbonate of lime, probably in the form of aragonite, in an organic matrix produced by the disintegration of cells which I have described as calicoblasts." It is further characterised by its blue colour. A peculiar form of the axial skeleton (Fig. 155), consisting of alternate nodes mainly composed of keratin, and internodes mainly composed of calcium carbonate, is seen in the families Isidae and Melitodidae. In the Melitodidae the nodes contain a considerable number of loose spicules, and the internodes are mainly composed of spicules in close contact but firmly cemented together by a sparse horny matrix. In the Isidae the scanty calcareous substance of the nodes, and the bulk of the substance of the internodes, is formed of amorphous crystalline limestone.
The Alcyonaria exhibit a great variety of colour. Very little is known at present of the chemistry of the various pigments found in the group, but they may conveniently be arranged in two sections, the soluble pigments and the insoluble pigments. To the former section belong various green and brown pigments found in the anthocodiae and superficial coenenchym of many genera. These are related to chlorophyll, and may be very largely the product, not of the Alcyonarians themselves, but of the symbiotic "Algae" (cf. p. [261]) they carry. A diffuse salmon-pink colour soluble in spirit occurs in the living Primnoa lepadifera of the Norwegian fjords, and a similar but paler pink colour occurs in some varieties of the common Alcyonium digitatum. Gilchrist[[366]] states that when he was preserving specimens of Alcyonium purpureum from Cape waters a considerable quantity of a soluble purple pigment escaped.
But the predominant colour of Alcyonarians is usually due to the insoluble pigments of the calcareous spicules. These may be of varying shades of purple, red, orange, and yellow. The colours may be constant for a species or genus, or they may vary in different specimens of one species, or even in different parts of a single colony. Thus the skeletons of Tubipora musica from all parts of the world have a red colour, the species of the genus Anthomastus have always red spicules. On the other hand, we find in Melitodes dichotoma red and yellow varieties in the same locality, and in M. chamaeleon some of the branches of a colony are red and others yellow. In Chironephthya variabilis the colour of the spicules in any one specimen varies considerably, but in a collection of several specimens from a single locality a kaleidoscopic play of colours may be seen, no two specimens being exactly the same in the arrangement of their colour pattern. The influences that determine the colour of the spicules is at present quite unknown, and in view of the great variability that occurs in this respect, colour must be regarded as a most uncertain guide for the determination of species. The blue colour of the genus Heliopora is due to a peculiar pigment which shows characteristic bands in the spectrum.[[367]]
Phosphorescence.—A great many Alcyonaria are known to be phosphorescent. Moseley says that "All the Alcyonarians dredged by the 'Challenger' in deep water were found to be brilliantly phosphorescent when brought to the surface." The phosphorescence of the common British Pennatula phosphorea has attracted more attention than that of any other species, and has been well described by Panceri, Forbes, and others. Forbes[[368]] says, "The pen is phosphorescent only when irritated by touch; the phosphorescence appears at the place touched, and proceeds thence in an undulating wave to the extremity of the rachis, but never in the opposite direction; it is only the parts at and above the point of stimulation that show phosphorescence, the light is emitted for a longer time from the point of stimulation than from the other luminous parts; detached portions may show phosphorescence. When plunged in fresh water, the Pennatula scatters sparks about in all directions—a most beautiful sight."
Panceri was of opinion that the mesenteric filaments were the organs of phosphorescence, but the whole question of the cause and localisation of the light in these colonies requires further investigation.
Food.—Very little is known about the food of Alcyonaria, but it is very probable that it consists entirely of minute larvae and other living organisms. When the coelenteric cavities of preserved Alcyonaria are examined, food is very rarely found in them, although fragments of Crustacean appendages have occasionally been seen in the neighbourhood of the mesenteric filaments. Experimenting upon Alcyonium digitatum, Miss Pratt[[369]] has found that the zooids seize and swallow various small organisms of a surface-net gathering, and that they will also swallow finely minced fragments of the muscle of fish, but that they reject many kinds of fish ova. In many tropical and some extra-tropical species the superficial canal systems and the inter-mesenterial spaces of the zooids contain a large number of Zooxanthellae, and their presence seems to be associated in some cases with a decided degeneration of the digestive organs. It has been suggested that these symbiotic "Algae" prepare food materials after the manner of plants, and that these are absorbed by the hosts, but it appears improbable that in any case this source of food supply is sufficient. It must probably be supplemented in some degree by food obtained by the mouth, and digested in the coelenteric cavity.