CCELENTERATA : ALCYONARIA. 167 



In Xenia the colony is branched, and the polypes are non- 

 retractile ; and in Anthelia and Sympodium the actinosoma 

 has the form of a membranous crust attached to foreign 

 bodies. Lastly, in Sarcophyton (as shown by Moseley) the 

 colony consists of reproductive zooids, which have generative 

 organs and tentacles, and of sexless zooids, which have neither 

 of these organs, but possess a mouth and stomach-sac. 



FAMILY II. TUBIPORID^E. In the Tubiporidce, or " organ- 

 pipe corals," of which T. musica (fig. 80) is a familiar example, 



Fig. 80. A, Portion of the corallum of Tubipora musica, of the natural size, showing 

 the tubular corallites and their connecting floors. B, Polype of the same, greatly en- 

 larged, showing the mouth and tentacles. 



there is a well-developed sclerodermic corallum, with theca, but 

 without septa. The corallum is composed of a number of 

 bright-red, tubular, cylindrical thecae, which are united toge- 

 ther externally by horizontal plates or floors, which appear to 

 be formed by periodical extensions from the mouths of the 

 tubes. The polypes are usually bright green in colour, and 

 possess eight tentacles each. 



As shown by Prof. Perceval Wright, the tubes of Tubipora 

 are in reality composed of fused spicules ; and the polypes 

 when alarmed retract themselves within their tubes, the upper 

 portions of which are composed of loose fusiform spicules, and 

 are thus capable of withdrawal into the lower dense portion of 

 the thecae. 



FAMILY III. PENNATULID^E. The Pennatulidtz, or " Sea- 

 pens," are defined by their free habit, and by the possession 

 of a sderobasic, rod-like corallum, sometimes associated with sclero- 

 dermic spicules. 



Pennatula, or the " Cock's-comb," consists of a free cceno- 

 sarc, the upper end of which is fringed on both sides with 

 feather-like lateral pinnae, which bear the polypes ; whilst its 



