ANTHOZOA ALCYONARIA. 



731 



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 in- 

 growth 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 ir- 

 regular 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 de- 

 generates 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. Corallium 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 . 



1 See 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. 



