CCELENTERATA : ALCYONARIA. 



171 



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 



Fig. 84. A, Colony of Heliopora cterrilea, of the natural size. B, Portion of the surface 

 of the same, enlarged, showing the apertures of the larger and smaller zooids. C, 

 Vertical section of a few of the tubes of the same, enlarged, showing the tabula. 

 (After Dana.) 



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 completely retractile, with 

 eight pinnately-fringed tentacles, and eight mesenteries. The 

 mesenteries, however, have no correspondence with the septa, 

 which are twelve in number as a rule. The septa are thus 

 seen to be pseudo-septa, and they cannot be regarded as being 

 homologous with the septa of the Zoantharia sderodermata. 

 The so-called coenenchymal tubes are occupied by sacs lined 

 by the endoderm, which are closed externally, but communicate 

 freely with the body-cavities of the polypes by means of trans- 



