54 SIDERASTREA RADIANS. 



from the united edges of the primary and secondary septa. Others of the 

 projections seem, however, to be quite independent of the septa. Secondary 

 calcareous matter is deposited among the projections, and this gives the more 

 compact character to the structure as a whole. The section represented on 

 plate 10, fig. 65, shows the relationship clearly. The columella is here 

 quite solid and distinct, and is structurally the same as the septa united with 

 it, having its own centers of calcification. In sections a little higher it is 

 spongiforui, as the trabeculas are free from one another. 



With such structural details alone available it is practically impossible 

 to say whether the columella of Siderastrea is a true or a false columella 

 (pseudocolumella) as these terms are understood in coral literature. Miss 

 Ogilvie (1897, p. 179) describes it as a "paliform pseudocolumella." A true 

 columella is considered to arise as an independent structure from the middle 

 of the basal plate, though the septal edges may secondarily unite with it. A 

 false or pseudocolumella, on the other hand, is an irregular skeletal tissue 

 formed from union of the inner or central ends of the septa, sometimes bound 

 together by a secondary deposit. Recourse to the developing coralla shows 

 the true nature of the columella in S. radians. As described later, and 

 illustrated by the figures on plates 4 and 5, independent skeletal upgrowths 

 are first formed from the basal plate, but later come into intimate relation- 

 ship with similar formations at the edges of the septa, and afterwards the 

 two groups are united into a solid, compact column by a deposit of calcareous 

 matter. 



Developrnentally, therefore, the columella of Siderastrea is a true colu- 

 mella, compact all the way, or compact below and spongy above. 



Histologically the columella usually shows two or three large circular 

 trabeculae arranged in a row and surrounded by a number of smaller tra- 

 beculas (plate 10, fig. 65). The larger probably represent the true elements 

 of the columella, which arise directly from the basal plate, while the smaller 

 are the septal teeth which also take part in its constitution. 



Viewed from the surface the columella is usually nearly circular in 

 outline and affords no aid in determining the principal axis of the corallite ; 

 but in sections some distance below the surface it generally presents a longer 

 and a shorter axis, and the former evidently corresponds with the directive 

 or principal axis of the corallite. To each extremity of the longer axis a 

 septum of the first order is attached, and the two are, no doubt, the directive 

 septa, though, owing to the difference in the number of septa on each side, 

 they are not always in the same plane (plate 10, fig. 64). The oval columella 

 may thus afford an important aid in the orientation of the septa and of the 

 corallites in a colony. 



