STKUCTUKE AND RELATIONS OE CERTAIN CORALS. 
99 
The transverse partitions in the tubes and calicles give evidence in their structure that 
they are later additions to the inside of an already formed tube. They are not merely 
transverse floors, but flat-bottomed cups of tissue fitted inside the old tube, and thus 
narrowing its bore considerably in the region where they become formed. The old 
boundary line of the tube below the tabula can nearly always be traced, continuing its 
course for some distance beyond and above the tabula (Plate 9. figs. 11, 15). The 
tabulse of the coenenchymal tubes seem in all respects identical in structure with 
those of the calicles. 
The structures which form the centres from which the systems of hard tissue radiate 
have been called axes. They have the appearance of being canals in the hard tissue ; 
but this appearance seems to be fallacious. They appear to represent the points of 
junction of the walls of the opposed coenenchymal tubes where imperfect fusion has 
taken place between these walls, and the interspace has been filled with amorphous 
rather than fibrous calcareous matter. In some cases in transverse sections these axes 
appear as elongated spaces between the adjacent tubes, rather than central canals. The 
appearance of these axial structures is accurately represented in Plate 8. fig. 4. There 
is always a somewhat opaque, fine, granular area around them, which shows often a series 
of concentric zones. 
The opaque tissue surrounding the axes is continued into the projecting points at the 
surface of the coral. These points sometimes show a banded appearance, as if they had 
received in growth successive caps of hard tissue (Plate 9. fig. 11, P). 
On the Growth of the Corallum of Heliopora caerulea. 
If a rapidly growing tip of a frond of Heliopora ccerulea be carefully protected from 
injury and macerated in potash, the appearance of its corallum will be that given in 
Plate 9. fig. 16. The tissue at the actual tip is seen to be much more delicate and 
spongy-looking than in the older parts. It consists here superficially of an aggregation 
of thin-walled cells, which are mostly multiangular injoutline at their mouths, some- 
times hexagonal, often pentagonal, often with curved sides, assuming these various 
forms apparently from mutual appressure in growth. In the angles where the walls 
of the adjoining cells meet are the commencements of new cells, which in their very 
earliest stage are often triangular in superficial outline’ (see diagram). Amongst this 
mass of polygonal cells new calicles are developed by the 
arrest in growth of one or more cells after they have reached 
a certain small height, which cell or cells form a central floor 
to the calicle. All around this central floor, contiguous, 
deeper, older cells form a circular zone. Their inner walls, 
i. e. those nearer to the centre of the growing calicle, cease 
to grow, and only their outer ones continue to develop ; and 
these being fused together form the lateral walls of the calicle, 
mdccclxxvi. p 
