108 THE VOYAGE OF H.M.S. CHALLENGER. 
acids than the older portions. If one of these small points, after having been treated 
with a strong solution of potash, be examined under the microscope, it will show appa- 
rently no trace of consisting of anything but the usual doubly refracting calcareous 
matter. If it be then slowly decalcified, an investing layer of finely fibrous tissue is 
gradually brought into view as the lime is removed. The fibrous tissue seems to form 
an investment to the hard part, or rather to be present only in its peripheral regions, the 
central part of the piece of corallum appearing to be free, or almost so, from contained 
fibrous structures, and thus to be more rapidly attacked and decomposed by the acid. 
In specimens of Heliopora which have been slowly decalcified in chromic acid, the 
appearance presented by one of these growing points as viewed from below is shown in 
Plate I. fig. 6. Here it will be seen that a mass of tissue composed of extremely fine 
fibres (B) occupies the space immediately within the layer of connective tissue cells. The 
fibres composing the mass are disposed in a concentric manner, externally around the 
centre of the mass, and more internally around two rounded cavities situate side by side 
in its centre. Appearances similar to this are presented by a section from the surface of 
Heliopora, prepared as described, cut parallel to the surface and viewed from beneath, 
sometimes two and sometimes one cavity appearing in the fibrous mass. The fibrous 
masses occupy the position which in the undecalcified coral is occupied by the projecting 
points of the corallum, and are identical in structure with the small investment of fine 
fibrous tissue which, as above described, can be obtained from a growing point of the 
corallum by decalcification. But the quantity thus derived from a portion of the 
corallum cleaned with potash is very small indeed in proportion to such a mass as that 
shown in Plate I. fig. 6. The spaces A, B shown in this figure were probably occupied 
by the central parts of two newly-formed excrescences on a projecting point of the 
corallum, whilst the hard tissue was extended thence for some distance amongst the 
fibrous tissue. It is, however, uncertain how far this extension reached. I have not 
been able to prepare such sections of hard and soft parts in contact as permit the elucida- 
tion of this question. 
[ have not seen the finely fibrous tissue in the deeper parts of the coral; but 
in some preparations traces of residual tissue are to be recognised in longitudinal 
sections occupying the former sites of parts of the corallum situate at some distance 
from the surface, as at Plate Il. fig. 4, P; but it does not here show the fibrous 
structure. 
It seems probable that the layer of connective tissue cells produces the finely fibrous 
tissue, and that within this tissue the calcareous matter is deposited gradually from 
within outwards, the tissue gradually bemg removed and absorbed as the process con- 
tinues. The finely fibrous tissue may be termed calciferous. Exactly similar tissue, with 
similar concentric fibrillation, occurs in similar relations in Pocillopora, though in this 
latter case the connective tissue cells are perhaps absent. 
