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 cpiestion. 



I 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 II. 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 being 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 Pocillopom, though in this 

 latter case the connective tissue cells are perhaps absent. 



