REPORT ON CORALS—HELIOPORIDZE. 105 
The fine structure of the hard tissue of the corallum of Heliopora is in many respects 
similar to that of the coralla of Hexactinian 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.€., 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 jom. The fibres are disposed more or less in laminz 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 ccoenenchym. 
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 laminz is to be remarked. In the 
corallum of Pocillopora 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." 
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 (PI. II. figs. 5, 9). 
The tabulz of the cconenchymal 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 ccenenchymal 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 Pocillopora is composed. The exact nature of the radial strie 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 as to form lamin. 
(ZOOL, CHALL, EXP.—PART viI.—1880.) G 14 
