HELIOPORIDiE. 



333 



the genus Heliopora. The rods are generally placed, each, at the 

 junction of four siphonopores, and are, therefore, essentially quad- 

 rilateral. They are firmly united with one another along their 

 opposed sides by dentated sutures, and they are excavated along 

 their angles by the siphonopores. Each rod possesses a single or 

 multiple, generally elongated, apparently structureless axis, round 

 which the calcareous tissue is disposed in radiating plates of great 

 tenuity, which look in cross-sections (fig. 215, a) like fibres. The 

 peculiar prismatic rods just described may be regarded as modified 

 spicules, which become laterally anchylosed. In the fossil species of 



ta 



Fig 215. — A, Tangential section of the corallum of the recent Heliopora ccerulea, enlarged 

 twenty times ; b, Longitudinal section, similarly enlarged, an, Autopore ; si, Siphonopore ; a, 

 Tube of a parasitic Annelide {Leucodora); s, Suture between two adjoining spicules; ta, Tab- 

 ula. (Original.) 



Heliopora the skeletal structure is commonly largely affected by 

 mineralisation, but in some cases (e.g., in H Blainvilleana) the 

 minute structure of the corallum can be shown to be essentially 

 similar to that of the recent H. ccerulea. Even where obliterated 

 by crystallisation, the minute structure can be inferred to have been 

 spicular, as the surface always shows the characteristic projecting 

 papillae formed by the free ends of the skeletal rods. 



It is worth noting in connection with the skeleton of Heliopora that 

 the corallum in the recent H. ccsrulea is commonly traversed by numer- 

 ous Annelide-tubes belonging to a species of Leucodora. These tubes 

 are intermediate in size between the autopores and siphonopores (fig. 

 215, a), are very regularly distributed, and have their mouths flush with 

 the general surface ; while they are not mere borings, but are coated by 



