ANTHOZOA ZOANTHARIA. 739 



become independent, or remain in continuity over larger or smaller 

 regions. As the coral grows, portions of the skeleton often become exposed 

 to a variable extent, and the exposed surface, like the exposed surface of a 

 simple coral, is attacked by the water, and by organisms, vegetable or animal. 

 The calycles are in the majority of colonial corals connected by a calcareous 

 coenenchyma or common skeleton, which is either perfectly solid, or ex- 

 cavated by internal spaces left during growth, but not connected with the 

 calycles nor open in the macerated state ; or it is traversed within and without 

 by tubular channels which lodge in the living condition canals connecting 

 the gastric cavities of the zooids, and therefore in the macerated skeleton 

 open superficially. Hence the division of Madreporaria into two great 

 sections, the Aporosa and the Perforata. The skeleton itself is composed of 

 calcareous ellipsoids, themselves made up of typical rhombs of Calcite. 

 Little is accurately known as to its development. In Astroides calycularis 

 it appears first as a calcareous ring beneath the base of the solitary larva 

 and the foreign object to which it is fixed. The ring becomes a disc, and 

 on the upper surface of the disc are formed twelve radial ridges, the first 

 septa, which bifurcate at their outer extremities. They correspond to 

 vertical folds of the base of the zooid, within the hollow of which they are 

 laid down. It is a disputed point whether or not the wall or theca arises 

 by a fusion of the outer ends of the septa (von Koch) or is a ring-like 

 thickening of the base independent of the septa (de Lacaze Duthiers). The 

 epitheca appears at the spot where the ectoderm of the base passes into the 

 ectoderm of the wall of the zooid. Of the twelve septa, six grow more 

 rapidly at first than the remaining six, a difference afterwards equalised. 

 A second series of twelve septa, and then a third of twenty-four appear. 

 The young Astroides begins to bud and form colonies. In the larger 

 calycles the second set of twelve septa become equalised with the first 

 twelve, and a fourth series of forty-eight septa is intercalated. An even 

 and regular development of septa in cycles is probably the rule: but (i) 

 cycles may become equalised, and (2) irregularities affecting a part or the 

 whole of a calycle may occur. Hence there are many difficulties to be 

 encountered in interpreting the skeleton of corals 1 . Moreover ' nearly allied 



1 Von Koch has recently given an account (M. J. xii. (i) 1886) of the morphology of the 

 Madreporarian skeleton according to his latest views. He regards (i) the epitheca as formed on the 

 outer surface of the wall of the zooid, (2) the theca as concentric with it, and (3) the two structures 

 as separated by spaces crossed by mesenteries and continuous with the intra-calycular portions 

 of the inter- and intra-septal chambers at the free edge or lip of the theca. The theca and 

 epitheca are, however, never separated, so far as the specimens and descriptions accessible to me 

 show, by anything more than a minimal space, and there is no evidence for the existence of soft parts 

 between them. The epitheca appears to be secreted by the free edge of the soft parts or limbus 

 covering the theca. At present it is by no means a settled point whether or no the theca may 

 grow in such a way that it cuts the mesenteries, as von Koch contends it does, into an extra- and 

 an intra-thecal portion, or whether the edge or limbus of the zooid simply overlaps the edge of the 

 theca. It is probable that both modes of growth occur. Mr. G. C. Bourne informs me that in 



3 B 2 



