III. ANTHOZOA 



229 



The theca arises by a fusion of sclerosepta. If this fusion takes place some 

 distance inside the peripheral ends of the sclerosepta, the distal ends of these 

 project on the outer surface as costce. Still outside these may be a second cup, 

 the epitheca. In the centre may occur a large calcareous column or several 

 smaller ones, the columella. As the polyps grow they build the thcca? higher and 

 higher and consequently draw out from the deeper 

 portions, which may become cut off by horizontal 

 partitions, the tabulce. Such tabulae occur in some 

 Madreporaria, Octocorallans, and Millepores (p. 

 217) which were formerly united in a group Tabu- 

 latas. 



It was once thought that the coral was a cal- 

 cified portion of the soft parts and hence that 

 sclerosepta were hardened sarcosepta, etc. This 

 has been disproved. The sclerosepta are formed 

 in the radial chambers between the sarcosepta, 

 and the theca inside and at some distance from 

 the column, the outer surface of which secretes 

 only the inconstant epitheca (fig. 199). From 

 the above it would appear that the sclerosepta 

 correspond in number to the sarcosepta, but this is not always the case. Thus 

 the Helioporidae, which on the grounds of the skeleton were regarded as Hexa- 

 coralla, are shown by the soft parts to be undoubted Octocoralla. 



By means of their skeletons the Anthozoa produce the well-known coral 

 reefs. When the reef reaches the surface it produces an island, the most note- 



FIG. icj8. Sderophyllia 

 margariticula Rafter Klunz- 

 inger). 



FIG. 199. FIG. 200. 



FIG. 199. Diagrammatic section of the flesh and coral of a hexacorallan; above the 

 line the section passes through the oesophagus, s; below the line it is lower down; r, 

 directives; coral black. 



FIG. 200. Diagram of relations of soft parts to coral (after Pfurtscheller). Shows 

 beginning sclerosepta and theca. 



worthy form being the atoll, a ring-like structure with a central lagoon. The 

 origin of these atolls, as well as that of fringing and barrier reefs, was for a long 

 time explained by Darwin's and Dana's theory of coral reefs. Later invcsiiga- 

 tions, notably those of Mr. Agassiz, afford another explanation. 



