The surface of the skeleton is covered superficially by a layer of cells or calycoblasts from which the hard structures are derived 1.
1Alcyonium digitatum sometimes occurs in free ball-like colonies.
1 The axial skeleton of Pennatulidae is contained within a septum which separates a dorsal from a ventral axial chamber, both in communication with the siphonozooids. The septum in question is produced, according to Wilson, in Renilla by a growth of endoderm cells from the aboral extremity of the primitive zooid. It consists of an axial and two superficial layers of cells continuous at the growing edge of the septum. Smooth oval calcareous spicules appear in the axial cells at an earlier date than the acicular spicules of the deep ectoderm cells. An adult Renilla has no axial skeleton, but Wilson's observation suggests that the epithelium covering the axial rod of other Pennatulids and the rod itself are of endodermic origin.
The dorsal and ventral chambers of the peduncle in Renilla are lined by endoderm, beneath which is a layer of longitudinal and circular endodermal muscles. Longitudinal muscles occur also in the septum.
See Wilson, Ph. Tr. 174, 1884, pp. 765-771, and pp. 776-782.
8 The facts detailed in this account of the skeleton are chiefly taken from von Koch. The skeletal elements may be classified as follows:
(1) Exoskeletal: horny cuticle of Clavularia, etc.; axial skeleton of Axifera, in which the skeletal epithelium is in continuity with the basal ectoderm.
(2) Mesoskeletal, (a) of ectodermic origin; spicules; their horny sheaths; axial skeleton of Pseudaxonia where no epithelium is present; tubes of Tubipora; calcareous axis of Cor allium; calcareous lamellate skeleton of Heliopora;
(b) of endodermic origin: axial rod of Pennatulidae. It is a mistake to consider the spicules, etc. as exoskeletal because the cells which give origin to them are of ectodermic descent. The fact that the cells are detached and wander into the mesoglaea deprives them of their ectodermic character.
Skeletal structures to be noted are the external and internal tabulae of Ttibiporidae, the septa and tabulae of Helioporidae. The external tabulae ( = platforms) of Tubipora are developed as rims close to the oral extremities of the zooids. These rims either surround neighbouring tubes or fuse with adjacent rims. They are at first soft and consist of ectoderm with a core of mesoglaea, but as they increase in size endodermic canals spring from the gastric cavities of the zooids and ramify in the mesoglaea. Spicules are formed at the same time and gradually unite into a firm skeleton. The internal tabulae are formed by a shrinkage, at the level of the platforms, of the endoderm and the lamina of mesoglaea lining the skeletal tube and the simultaneous formation of layers of spicules on the surface of the mesoglaeal lamina. These tabulae assume various shapes. The calcareous septa of Heliopora are ridges projecting inwards at the mouths of the calycles, usually twelve in number, but sometimes more, up to sixteen. The internal calcareous tabulae or horizontal floors are flat-bottomed cups added within the cavities of the calycles and tubes.
In both genera the internal tabulae limit the deep dying or dead region of the colony from the superficial and growing region.
As to the zooids themselves, when the mesoglaea of the colony is plentiful their anterior or oral extremities are 'invaginable; when it is scanty they are usually only contractile. The tentacles are hollow and set one over each of the eight perigastric chambers; they are pinnate, and are sometimes invaginable as in Cor allium and Heliopora, but are usually only contracted when the fore-part of the zooid is invaginated. The oesophagus or stomodaeum is of some length; its walls are transversely folded in invagination. The siphonoglyphe is absent in all the Axifera hitherto examined, probably also in the non-colonial genera. It is well developed in the Alcyouidae, feebly however in the autozooids of Sarcophyton. It is wanting in the autozooids of Pennatulidae, of Heteroxenia, and Paragorgia, but present in their siphonozooids. The mesenteries are thin. The retractor muscles, which are usually well-developed, are borne upon the homologous aspect of each mesentery, in such a way that the two mesenteries which limit a chamber corresponding to one extremity of the mouth have the muscles on the surfaces turned to each adjoining lateral chamber, whilst the two limiting the chamber at the opposite extremity of the mouth, have them on the surfaces turned to its cavity.
The first is termed the dorsal chamber, the second the ventral. The three lateral chambers on each side have consequently each a single muscle projecting into their cavities from the surfaces of the mesenteries turned towards the ventral aspect of the zooid. The two mesenteries inclosing the dorsal chamber are frequently of great length, and in the bud they usually develope more rapidly than the remaining mesenteries. The two mesenterial filaments of the dorsal mesenteries are composed of a border or ridge of high columnar cells, each with a single powerful cilium backed by flattened endodermic cells such as cover the surfaces of the mesenteries. The columnar cells are derived from a down-growth of the lower edge of the oesophagus; they are therefore ectodermic. Their cilia always produce an ascending currentl. The mesenterial filaments of the remaining mesenteries are composed entirely of endoderm cells which become columnar and for the most part glandular, but possess a single cilium. Thread cells of minute size occur among them. They develope before the dorsal filaments in the zooid originating from the egg. The sexes appear to be separate as a rule, and the colonies are even of one sex.
In Coral-lium, however, hermaphrodite individuals may occur, and the zooids of a single branch or of neighbouring branches in a colony may be of different sexes. The genital products are attached to the faces of a mesentery and possibly of particular mesenteries, in pedunculate capsules, always few in number. They are derived from the endoderm.
1 The calycoblasts of Heliopora give origin, according to Professor Moseley, to a fibrilloid organic substance which undergoes calcification, and is afterwards removed (?), seeing that little or no organic matter is to be detected in the older parts of the skeleton. Judging from Prof. Moseley's figures and description, the calycoblasts are of undoubted ectodermic origin.