REPORT ON CORALS — HELIOPORID^. 105 



The fine structure of the hard tissue of the corallum of Heliopora is in many respects 

 similar to that of the coralla of Hexactiuian corals. It is composed of doubly refracting 

 calcareous matter, which has a half-crystalline, half-fibrous structure. On transverse 

 section (PL I. fig. 4), it is seen to be made up of a series of systems of radiating fibres, 

 i.e., areas of calcareous tissue showing a radiate fibrous structure. In each system the 

 fibres radiate from a central axis, and diverge to fuse at the margin of the system with 

 the margins of the contiguous systems, a suture-like line being often observable where two 

 systems join. The fibres are disposed more or less in laminse which overlap one another. 

 The radial fibrous structure is to be seen only in thin slices or fragments of the coral 

 viewed by transmitted light. The fracture of the coral is irregular and crystalline. The 

 central axes of the systems correspond to the centres of the vertical beams already de- 

 scribed, which are prolonged above on the surface of the coral into papilliform projections. 

 In a vertical section of the corallum (PI. II. fig. 5, D), these axes are seen to take a vertical 

 course within the beams and branch beneath the newly formed buds of the ccenenchym. 

 The fibres are seen starting from the axes, spreading right and left from them throughout 

 the tissue with a uniform inclination upwards (i.e., towards the surface of the corallum). 

 In the plates forming the sides of the tubes (PL II. fig. 5, B) the sutures between the 

 fibres meeting one another at an angle from the two systems are well marked. The 

 appearance of a portion of the hard tissue, as seen under a high power, is shown in Plate 

 II. fig. 6, where the appearance of the overlapping laminse is to be remarked. In the 

 corallum of PociUopora definite rod-like prisms with polygonal ends are seen to exist 

 when these structures are viewed end on ; in Heliopora such a definite structure 

 apparently does not occur. 1 



The transverse partitions in the tubes and calicles give evidence in their structure that 

 they are later additions to the insides of already formed tubes. They are not merely 

 transverse floors, but flat-bottomed cups of tissue fitted inside the old tubes, and thus 

 narrowing their bore considerably in the region where they become formed. In nearly 

 all instances the old boundary line of the tube below the tabula can be traced, and is seen 

 to continue its course for some distance beyond and above the tabula (PL II. figs. 5, 9). 

 The tabulae of the ccenenchymal 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 here been called axes. They have the appearance of being canals in the hard tissue, 

 but this appearance seems to be fallacious. They probably represent the points of 

 junction of the walls of the opposed coenenchymal tubes where imperfect fusion has taken 



1 The radiating components of the hard tissue are here spoken of as fibres to distinguish them from these well- 

 marked prisms of which the hard tissue of Pocillo2)ora is composed. The exact nature of the radial striae seen in the 

 tissue of the Heliopora I do not understand ; they seem to represent spaces between variously shaped splinters, as it 

 were, of hard matter arranged so us to form lamina?. 



(ZOOL. CHALL. EXP. PART VII. 1880.) G 14 



