220 



RUGOSA AND ALCYONARIA. 



essentially similar to Heliopora in structure. We thus are 

 presented with a new family of the Alcyonaria that of the 

 Helioporidce which has played no inconsiderable part in 

 geological history. The Helioporidce were formerly placed 

 in. the " Tabulate " section of the Zpantharia sclerodermata, 

 and possess a ^vell-developed sclerodermic corallum, composed 

 of tabulate tubes of two sizes, the larger ones being furnished 

 with rudimentary septa. In the living Reliopora ccerulea 

 (fig. 115) the corallum is composite and sclerodermic, and is 



Fig 115. A, Portion of the corallum of Heliopora ccerulea, of the natural size (after Milne- 

 Edwards); B, Portion of the surface of a branch of Heliopora ccerulea, magnified eight 

 diameters (after Moseley) ; c, c, c, the openings (" calices ") of the corallites, surrounded by 

 the smaller tubes of the coenenuhyma. 



composed of corallites united by what has usually been 

 regarded as a " ccenenchyma." The corallites are tubular, 

 crossed by well-developed tabulae, and having their walls 

 folded in such a manner as to give rise to a variable number 

 (generally twelve) of septal laminae. The coenenchyma, so 

 called, is composed of slender tubes, of smaller size than the 

 true corallites, packed closely side by side, crossed, like the 

 corallites, by regular transverse tabulae, but destitute of 

 septa. The soft parts occupy only the parts of the corallum 

 above the uppermost tabulae, and therefore only a surface- 

 layer of the colony is actually alive. The polypes are com- 

 pletely retractile, with eight pinnately-fringed tentacles, and 

 eight mesenteries. The mesenteries, however, have no cor- 

 respondence with the septa, which are twelve in number as 



