STEUCTUEE OF A SPECIES OF MILLEPOEA. 
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continuous and highly polished, and is exactly moulded on the surface of the glass, repro- 
ducing casts of the most minute splinterings or scratching^. 
In homology with this continuous layer, layers more or less continuous occur in the 
more massive coralla, appearing in vertical sections as lines of calcareous matter running 
parallel to the surface of the corallum, and indicating successive stages of growth ; and 
the tubercles of which the mass of the Tahitian Millepora is made up, when cut through 
vertically to the surface, show a series of such lines of growth following the contour 
of the surface. It is in connexion with these layers that are developed the successive 
transverse laminae or tabulae which divide the cavities of the calicles into a series of 
chambers (Plate 2. fig. 5). As the corallum is extended in growth at certain intervals, 
possibly after each period of generative activity, a tabula is formed, reducing the depth 
of the calicle and shutting off the living tissue from the abandoned dead structures 
below. The larger canals and their branches ramify in planes parallel to the surfaces 
of the corallum, being confined within each successively added thin layer of the corallum, 
and 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 (Plate 2. fig. 5). 
In the thin films of Millepora alcicornis the trabeculae of hard tissue run with remark- 
able uniformity in straight lines parallel to one another, whilst the main canals cross 
them with a serpentine course. 
Histological Structure of the Corallum. 
In histological structure the hard tissue composing the corallum of Millepora seems 
to resemble closely that of the coralla of Heliopora and most Anthozoa. It is composed 
of lamellae of fibro-crystalline calcareous matter (Plate 2. fig. 8), the fibres of the super- 
posed lamellae crossing one another at all angles in the mass. In some places, in thin 
sections of the corallum, the appearance shown at a (Plate 2. fig. 8) is clearly to be seen.* 
The calcareous fibres of the hard tissue terminate towards a cavity in the corallum 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 corallum was in active growth. The hard tissue is bored in 
all directions by parasitic vegetable organisms (Plate 2. figs. 6 & 8). 
Chemical Composition of the Corallum. 
Although the animals forming the corallum of Millepora differ so widely from those 
by which all other corals are secreted, their coralla appear to agree in chemical compo- 
sition with those of other corals as closely as they do in histological structure. Analyses 
of the coralla of two species of Millepora are given by Professor Dana. One is an 
analysis of Millepora tortuosa from the Fijis, by Mr. Lilli man, Jun.* The composition 
was found to be as follows : — 
* ‘ Structure and Classification of Zoophytes,’ by_ J. D. Dana, A.M. Philadelphia : Lea and Blanchaed, 
1846. Appendix, p. 130. 
