The autozooid always has the typical structure above given. The si-phonozooid differs from it in the absence of tentacles and retractor muscles; in the great development of the siphonoglyphe; in having the two dorsal, and sometimes the two ventral mesenteries, longer than the rest, the first named only having mesenterial filaments2; and in being sexless. Such zooids occur in Sarcophyton and Heteroxenia among Alcyonidae, and in Pennatulidae; among Pseudaxonia in Paragorgia and Siphonogorgia where they bear ova, and also in Corallium where it is stated that they develope into autozooids. Similarly certain zooids found in the Pennatulids Halisceptrum and Virgularia, which differ however from siphonozooids in having genital organs and traces only of the dorsal mesenterial filaments, develope into autozooids when they have discharged their sexual function.

1The only filaments present in Xenia and Sympodium are the two dorsal. Haacke, Z. A. vii. 1884.

2Wilson says (op. cit. p. 725) that the siphonozooids of Renilla have no mesenterial filaments. According to Professor M. Marshall (Trans. Roy. Soc. Edinburgh, xxxii. p. 145) the siphonozooids of Umbellula gracilis possess a single tentacle; and the larger siphonozooids of Pennatula phosphorea var. aculeata have a long abaxial process formed probably by elongated calyx teeth (op. cit. p. 126). A single large siphonozooid terminates the axis of the colony in Renilla. Water is constantly discharged by it; hence 'exhalent zooid' = Haupt-zooid of Kolliker. The other siphonozooids are small, clustered and inhalent.

The gastric cavities of the zooids of a colony are connected by systems of tubes variously arranged in different Alcyonarians, and lined by endo-derm. The buds originate as outgrowths from these tubes, with an ingrowth of ectoderm to form the stomodaeum. A definite arrangement of the zooids in a colony is frequently observable, their dorsal aspects being turned in the same direction.

The ectoderm may be unilaminar or, as on the tentacles, multi-laminar. It is ciliated in the latter position and in the stomodaeum. The endoderm is unilaminar. Its cells may be flattened or columnar, and in some instances at least, e. g. in Corallium, are ciliated throughout. The retractor muscles of the tentacles, oral disc, and stomodaeum are derived from it. The mesoglaea is more or less hyaline, sometimes fibrillate, and often contains stellate or branched cells.

The Pennatulidae are phosphorescent, and the phosphorescent material is contained in eight cords of cells attached to the gastric aspect of the stomodaeum of both auto- and siphono-zooids.

Development sometimes takes place within the parent, e.g. Sympodium, Corallium, Clavularia petricola, sometimes external to it, e. g. Alcyonium, Renilla; and the ova may be attached to the parent externally by a gelatinous material as in Clavularia crassa. Segmentation is very irregular and does not always extend at once to the centre of the ovum. In Renilla there is a small and transitory segmentation cavity, but as a rule the embryo is a solid mass of cells. The ectoderm is differentiated as a superficial layer: the endoderm arises from the central mass as a layer of cells underlying the ectoderm; the remainder of the mass degenerates and is absorbed. The oesophagus is a hollow, e. g. Sympodium, or solid, e. g. Renilla, ingrowth of ectoderm. The mesoglaea is derived from the base of the ectoderm cells, many of which pass into it; that of the mesenteries may originate from the endoderm. The tentacles are at first simple and conical. The spicules appear in cells derived from the ectoderm, and the central axis of Gorgonia is formed after the attachment of the larva as a small basal plate, which subsequently grows upwards as a papilla.

Few fossil Alcyonaria are known. The Tubiporidae are probably represented in Silurian, Devonian, and Carboniferous strata by Syringopora and its allies. Cor allium is found from Jurassic times onward; a Penna-tulid occurs in the upper Chalk, and the family Helioporidae has several representatives in Silurian and Devonian strata. The Favositidae from the same period are probably Alcyonarian 1

1See Moseley, Challenger Reports, ii. p. 121, and Hickson, Q. J. M. xxiii. pp. 569-74; cf. Nicholson, A. N. H. (5), xiii. pp. 29-34.

The classification of the Alcyonaria is by no means settled in details. The following groups however may be distinguished.

I. Non-Colonial Forms

Proto-Alcyonaria (Hickson). Haimeidae. Three genera only; Monoxenia, from Arabian coast; Haimea, from Fiji Islands j Hartea, from west coast of Ireland.

II. Colonial Forms

1. Cornularidae

Zooids originating from a tubular stolon, e.g. Clavularia, a band-like stolon, Sarcodictyon, or a basal expansion, Sympodium. They are usually free; but Clavularia viridis has connecting tubes between the zooids from which new zooids may spring.

2. Tubiporidae

Zooids originating from a basal disc and connected by platforms from which new zooids spring; spicules uniting to form a continuous system of tubes and platforms; Tubipora.

Hickson unites I and 2 into a group, Stolonifera.

3. Alcyonidae

Zooids forming a more or less massive colony; rarely dimorphic as in Sarcophyton and Heteroxenia; no skeletal structures save scattered calcareous spicules; gastric cavities of zooids remarkably long; Alcyonium, etc.

Coelogorgia, from Zanzibar, which forms branching colonies with tubular axes, and one or two allied forms perhaps belong here.

4. Pseudaxonia

An axial branched skeleton present, derived entirely from cells of the mesoglaca. This axis consists of fused calcareous spicules in the Corallinae, e. g. Corallium rubrum; of calcareous spicules cemented by horny lamellae in the Sclerogorgiacea; of alternating joints, one set composed of spicules united by horny lamellae, the other of fused spicules, Melitheacea, i.e. Melithea and Mopsea.

Siphonogorgia and Paragorgia with a spicular axis form a transition to 3. They have dimorphic zooids; so too probably Corallium.

5. Axifera

An axial branched skeleton present, derived from a layer of ectoderm cells invaginated from the base of the colony; skeleton lamellate, horny, or horny and calcareous; in Isis composed of alternating horny and calcareous joints; Primnoa, Gorgonia, etc.

6. Pennatulidae

Colony free, with a base or peduncle sunk in sand or mud. Zooids dimorphic; confined to the exposed part of the colony. An axial skeleton generally present, covered by an epithelium, probably of endodermic origin. Some are phosphorescent. Virgularia, Funiculina, Pennatula, Renilla, Umbellula, etc.

7. Helioporidae

A calcareous skeleton composed of lamellae of calcite, as in the Madreporarian Zoantharia. Zooids dimorphic. Siphonozooids reduced to short closed tubes. The calcareous cups for the zooids are closed below by horizontal tabulae, which are formed successively during the growth of the coral, one at some little distance above another. Heliopora, from the Philippine Islands.

General literature. Milne Edwards and Haime, Histoire Nat. des Coral-liaires, 3 vols., Paris, 1857-1860. Klunzinger, Korallthiere des Rothen Meeres, (with lit.) 3 pts., Berlin, 1877-1879. On the skeleton, von Koch, Biol. Centralblatt, ii. 1882-83.

Monoxenia, Haeckel, Arabische Korallen, 1876, p. 7; Hartea, P. Wright, Q. J. M. v. 1865; Haimea, Milne Edwards, Histoire Nat. des Coralliaires, Paris, i. 1857, p. 104.

Clavularia, Hickson, P. R. S. xl. 1886, p. 322; von Koch, M. J. vii. 1881. Sarcodictyon, Herdman, Proc. Roy. Phys. Soc. Edinburgh, Session 1883-84. Tubi-pora, Hickson, Q. J. M. xxiii. 1883. Sarcophyton, Moseley, Challenger Reports, ii.

1881, p. 117; Corallium, de Lacaze Duthiers, Histoire Nat. du Corail, Paris, 1864: cf. Moseley, Q. J. M. xxii. 1882. Isi's Neapolitana, von Koch, M. J. iv. 1878; Gorgonia verrucosa. Id. ibid.: various Axifera, Id. Mitth. Zool. Stat. Naples, iii.

1882. Pennatulidae, Kolliker, Challenger Reports, i. 1880: Id. Abhandl. Senckenb. Ges. vii. 1879; viii. 1872; Report on the Oban Pennatulidae, A. M. and W. P. Marshall, Birmingham, 1882; Pennatulida dredged by H. M. S. Triton, A. M. Marshall, Trans. Royal Soc. Edinburgh, xxxii. pt. 1, Session 1882-83; Umbellula, Kolliker, Festschrift zur Feier des 25-jahrigen Bestehens d. phys. med. Ges. in Wurzburg, 1874; cf. Lindahl, A. N. H. (4), xiii, 1874, and von Willemoes-Suhm, ibid. xv. 1875. Renilla, Wilson, Ph. Tr. 174, 1884. Heliopora, Moseley, Challenger Reports, ii. 1881.

Skeleton, von Koch, M. J. iv. 1878; Id. M. J. vii. p. 484; Id. Biol. Central-blatt, ii. 1882-83. Development in Gorgonia, Id. Mitth. Zool. Stat. Naples, iii. p. 550. Skeleton in Corallium, etc, Nicholson, A. N. H. (5), xiii. 1884.

Siphonoglyphe, Hickson, Ph. Tr. 174, 1883. Mesenterial filaments, Wilson, Mitth. Zool. Stat. Naples, v. 1884; ditto of Xenia and Sympodium, Haacke, Z. A. vii. 1884.

Phosphorescence of Pennatulidae, Panceri, Atti Ac. Napoli, v. 1873; cf. Q. J. M. xii. 1872.

Development of Renilla, Wilson, Ph. Tr. 174, 1883; of Clavularia and Sympodium, Kowalewsky and Marion, An. Mus. Nat. Hist. Marseilles, i. 1883.

Classification, von Koch, M. J. iv. 1878, p. 474; Hickson, Ph. Tr. 174. p. 699.