STEUCTUEE AND EELATIONS OF CEETAIN COEALS. 
101 
matter is formed, they are wanting, and homogeneous connective-tissue alone present. 
It seems hence almost certain that they are the instruments of formation of the cal- 
careous tissue. The newly formed and growing points of the corallum yield much 
more organic remains after treatment with 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 apparently no trace of consisting of any 
thing 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 Helioporci 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 8. fig. 6. Here it will be seen 
that a mass of tissue composed of extremely fine fibre (B) occupies the space imme- 
diately 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. Appear- 
ances similar to this are presented by a section of Heliojoora, 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 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, and 
bears no proportion to the mass shown in fig. 6. The spaces A, B 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, but how far is uncertain. I have not been able to prepare sections 
of hard and soft parts in contact which will show this. 
I have not seen the finely fibrous tissue in the deeper parts of the corallum ; but in 
some preparations it is to be recognized in longitudinal sections, as at Plate 9. fig. 10, 
P, though here not showing the fibrous structure. 
It seems probable that the layer of connective-tissue cells produces the finely fibrous 
tissues, and that within this tissue the calcareous matter is deposited from within out- 
wards, the fibrous tissue gradually being removed and absorbed. The finely fibrous 
tissue may be termed calciferous. Exactly similar tissue with similar concentric fibril- 
lation occurs in similar relations in Pocillojpora, though here the connective-tissue cells 
are perhaps absent. 
In no part of the growing points of the corallum of Heliopora is there any trace of 
the calcareous tissue being built up of the fusion together of a network of spicules, as 
p 2 
