332 CHARACTERS AND DIVISIONS OF ALCYONARIA. 



only are not at all constant in number, but do not correspond with 

 the mesenteries of the living polypes, these being always eight in 

 number. Hence, the radial ridges of Heliopora must be regarded 

 as " pseudosepta," and they are not homologous with the " septa " of 

 the Madreporarians. The " siphonopores " or interstitial tubes are 

 destitute of pseudosepta, and in the case of the living H. ccerulea 

 they are occupied by structures which Professor Moseley regards as 



Fig. 214. — A, Colony of the recent Heliopora ccprulea, of the natural size ; b, Portion of the 

 surface of the same, enlarged, showing the apertures of the larger and smaller zooids ; c, Ver- 

 tical section of a few of the " siphonopores" enlarged, showing the tabulae. (After Dana.) 



probably of the nature of rudimentary, 

 " siphonozooids " have the form of sacs 



sexless polypes. These 

 lined by the endoderm, 



closed externally, but communicating with the cavities of the auto- 

 zooids by means of canals in the soft tissues. 



As regards its minute structure, the skeleton of Heliopora con- 

 sists of fibro-crystalline carbonate of lime having a very peculiar and 

 characteristic arrangement. When examined in thin sections (fig. 

 215), the corallum is seen to be composed of radially disposed, par- 

 allel, prismatic rods, the apices of which project above the general 

 surface as prominent blunt papillae which occupy the angles of junc- 

 tion of contiguous siphonopores, and constitute a diagnostic feature of 



