106 THE VOYAGE OF H.M.S. CHALLENGER. 
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 the axial structures is accurately represented in Plate I. fig. 4. There is always a 
somewhat opaque, fine, granular area around them, which often shows a series of con- 
centric 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 (PI. II. fig. 5, P). 
Mode of Growth of the Corallum of Heliopora cerulea. 
If a rapidly growing tip of a frond of Heliopora caerulea be carefully protected from 
injury and macerated in potash, the appearance of its corallum will be that given in 
Plate IL. fig. 10. 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 in outline at their mouths, sometimes 
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 stages are often triangular in super- 
ficial 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. 
The arrested cell or cells form a central floor to the new calicle, 
around which lies a circular zone of contiguous, deeper, and 
older cells. The inner walls of these cells, 7.¢e., those nearer to 
Diagram illustrating the mode 
of growth of the corallum of ; 5 
Heliopora coerulea, outer ones continue to develop, and being fused together form 
the lateral walls of the calicle. The plications in the wall of 
the fully-formed calicle are to a great extent the result of this peculiar mode of growth ; 
but not entirely so, for sometimes in a young calicle two plications are present which 
may be seen to correspond to one lateral tube only. This will be understood by reference 
to Plate II. figs. 10, 11, and also to Plate II. fig. 9, where at B a section of a newly- 
formed calicle is given. 
the centre of the growing calicle, cease to grow, whilst their 
From the peculiarity of the mode of growth above described it results that in a 
newly-formed calicle the cavity is comparatively shallow in the centre, but is prolonged 
at the bottom all round into a series of tubular offsets. Into these tubular offsets the 
