ADULT COLONY. 19 



At the close of the process a delicate mass of algal filaments remains in the 

 spaces previously occupied by the corallum, but is readily removed. The 

 exposed polypal wall then reveals a smooth surface, very complicated in 

 form, but everywhere continuous. 



After removal of the skeleton the individual polyps are seen to be 

 wholly free from one another, except at the surface of the colony. No 

 other connection or communication between one polyp and another exists. 

 Each polyp appears as if made up of a deeply lamellated column, attached 

 above to a flattened layer which represents the column wall and disc. A 

 middle tubular space, formerly occupied by the columella, extends for some 

 distance upwards, when it terminates blindly. The lamellae represent the 

 tissues which occupied the interseptal spaces. They are now wholly free 

 from one another below, but in the upper region are united centrally. They 

 are easily torn apart along their central and upper lines of attachment. 

 One of the separated lamellae, slightly enlarged, is represented on plate 6, 



fig. 33- 



The whole of the external wall liberated by decalcification is the mor- 

 phological base of the polyp. Though columnar in form it in no way 

 corresponds with the column of the skeletonless Actiniaria, but represents 

 the flattened basal disc which, everywhere continuous, has become much 

 infolded and subdivided in correspondence with the upward growth of the 

 septa and columella. Thus the column of a decalcified retracted polyp is 

 really the vertically elongated, much subdivided basal disc. The true column 

 wall, corresponding with that of an actinian, is here represented by the 

 narrow periphery of the flattened or concave superficial disc which constitutes 

 the upper end of the polyp. To understand the form ultimately assumed 

 by the originally flat basal disc, the arrangement of the septa and columella 

 in the individual corallite must be borne in mind, as every part of these is 

 covered by the basal wall. An early stage in the basal complexity is 

 represented in the section of the young polyp on plate 9, fig. 53. 



The upper part of the liberated tissues appears delicate and somewhat 

 clear and transparent, but as the aboral termination is approached the walls 

 become denser and more opaque white. Such an alteration in the external 

 character of the embedded tissues is met with in nearly all corals, and is 

 usually more marked than in Siderastrea. It is found to be associated with 

 a corresponding thickening and histological modification of the endodermal 

 layer (p. 32). 



The polyps when set free are subpolygonal in outline, and, as already 

 mentioned, are wholly distinct from one another, except at their uppermost 



