20 THE VOYAGE OF H.M.S. CHALLENGER. 
never take a vertical course leading from the depths of the coral to the surface. A free 
vertical communication is, however, established by the smaller vessels (Pl. XIII. fig. 5). 
In the thin films of Millepora alcicornis the trabecule of hard tissue run with remarkable 
uniformity in straight lines parallel to one another, whilst the main canals cross them 
with a serpentine course. 
Histology of the Caenosteum.—In histological structure the hard tissue composing the 
coenosteum of Millepora seems to resemble closely that of the coralla of Heliopora and 
most Anthozoa. It is composed of lamellee of fibro-crystalline calcareous matter (PI. XIII. 
fig. 8), the fibres of the superposed lamellz crossing one another at all angles in the mass. 
In some places, in thin sections of the coenosteum, the appearance shown at a (PL XII. 
g. 8) is clearly to be seen. The calcareous fibres of the hard tissue terminate towards a 
=> 
cavity in the coenosteum as a series of short points, seeming to show a composition of the 
hard tissue out of definite rod-like elements. Such an appearance is only to be met with 
sparingly, and possibly occurs at spots where the ccenosteum was in active growth. The 
hard tissue is bored in all directions by parasitic vegetable organisms (Pl. XIII. figs. 
6 and 8). 
Chemical Composition of the Canostewm.—Although the animals forming the ccenos- 
teum of MWillepora differ so widely from those by which all other corals except the 
Stylasteridze are secreted, their ccenostea appear to agree im chemical composition with 
those of other corals as closely as they do in histological structure. Analyses of the 
coenostea of two species of Millepora are given by Professor Dana. One is an analysis 
of Millepora tortuosa from the Fijis, by Mr Silliman, Jun+ The composition was found 
to be as follows :— 
Carbonate of lime : : : . 94:226 
Phosphates and fluorides , 16200 
Organic matter 4°574 
Mr 8. P. Sharples’ found the cosnosteum of Millepora alcicornis to consist of— 
Carbonate of lime . : : . ee EAS 
Phosphate of lime . : : : een 
Water and organic matter : : ; 2°4 
There is no marked difference between these results and those obtained from 
Anthozoan corals. 
' Structure and Classification of Zoophytes, by J. D. Dana, A.M., Philadelphia, Lea and Blanchard, 1846, 
Appendix, p. 130. 
* Corals and Coral Islands, by J. D. Dana, London, 1872, p. 105. 
