190 W. I. Robinson, 



function should have been that of anchorage of the intern* 

 structures there might be no direct evidence of the attachmei 

 which evidently in modern corals is more a case of intimate 

 inter growth of the ectoderm and skeleton than an attachmei 

 in particular localities by ligaments or muscle endings. 



Ogilvie (1897:295) noted that in forms with a prominent 

 columella the lower part of the column and the circle of mesen- 

 tery ends form a trench, thus affording a somewhat more 

 sheltered situation for the gonads, which are attached to the 

 lower part of the mesenteries, than a flat-floored cavity would 

 offer. Whatever functions other than a central support or 

 anchorage the columella may have had, it invariably indicates 

 an invagination of the basal disk. It is a secretion in the ectoderm 

 and as such could never have pierced the basal disk and come 

 into direct contact with the internal organs. It has also been 

 suggested by Ogilvie that the columella of recent corals may be 

 looked upon as a specialization of the tabulae of Paleozoic forms. 

 Those early Paleozoic forms which had the least support from 

 below, such as Streptelasma and the Cyathaxonidse, seem to have 

 been especially liable to form columellas. On the other hand, 

 those forms with well developed tabulae, as in Zaphrentis, were 

 less liable to form them. In the late Paleozoic, columella forma- 

 tion seems to be more or less correlated with the development 

 of many septa as though it were representative of or the result 

 of crowding. 



Steps in Development. 



Siderastrea. — Duerden in his study of Siderastrea radians 

 (1904) has indicated the following steps in the development of 

 an essential columella: 



1. The columella is not seen until ten or twelve septa have 

 appeared. 



2. The first evidence of a central structure is the appearance 

 of a few granules or knobs on the smooth central area of the 

 basal disk. This may be accompanied by a thickening of the 

 ends of one or two septa or by the development of several spinose 

 growths from the septal ends. 



3. Later on more knobs and spinose growths appear and a 

 secondary infilling of calcareous matter makes the whole structure 

 compact. This process of calcification of the loose structure 



