STRUCTURE OR A SPECIES OR MILLEPORA. 
125 
radiating vessels. In vertical sections from osmic-acid preparations the muscular 
elements can be observed as isolated excessively fine fibres (as far as was seen, without 
nucleus), which appear to be distinct from the membranous layer of the zooid, though 
in close relation with it. These bundles of longitudinal fibres are plainly to be seen in 
preparations of all kinds, and no doubt it is to their presence that the zooids owe their 
power of almost instantaneous retraction. In several osmic-acid preparations an 
appearance indicating the existence of a set of circularly-directed fibres lying externally 
to the longitudinal fibres, or possibly of circular fibrillation of the membranous layer, 
was seen, but the existence of such structure was not determined with certainty. 
Structure of the soft parts of the Hydrophyton. 
The canals and spaces within the calcareous corallum are occupied, as has already 
been stated, by a network of soft tissue. This, together with the hard parts, constitute 
what, according to Allmax’s nomenclature, must be termed the hydrophyton, or 
common basis by which the several zooids of the colony are kept in union with one 
another.” In the case of Millepora the hydrophyton appears from its structure to be 
homologous with the hydrorhiza of other hydroids, the hydrorhiza here being extra- 
ordinarily developed, so as to form a thick network surrounding cavities into which the 
zooids are retracted, and forming by means of its ectoderm a massive calcareous 
structure. Only a thin layer at the surface of the coral is living. This layer separates 
from the underlying dead matter when the coral is decalcified in acids, and appears as 
a soft membrane about - 5 millim. in thickness. When the entire corallum is dissolved 
away there remains besides this membrane only a greenish gelatinous mass, which 
consists of the mycelium and spores of the parasitic organisms, which were the sole living 
occupiers of its deep parts. The living part of the hydroid seems to be entirely 
confined, as is the case in Heliopora ccerulea , to the region superficial to the last-formed 
tabulae. 
The soft parts of the hydrophyton consist of a series of ramifying canal-systems, 
which occupy in the recent condition the canals already described as existing in the 
corallum. The branches and secondary branches of the canals are joined by a complex 
network of smaller vessels, which join in all directions the body-cavities of the zooids 
(Plate 3. fig. 12), and thus maintain a vascular connexion between the various zooids of the 
colony of the freest character. In some cases comparatively large tertiary branches of 
the canals join the zooid-cavities directly. The large main canals run sometimes for 
long distances, and in a species of Millepora obtained at Zamboangan, Philippines, are 
plainly visible to the naked eye on the surface of the corallum, extending sometimes for 
as great a distance as 1^ inch. The ramifications of the hydrophyton are best seen 
on the under surface of the superficial living film decalcified in chromic acid and viewed 
by reflected light. The appearance presented in such a preparation is accurately repre- 
sented in Plate 3. fig. 12. The appearance of the soft parts of the hydrophyton, as seen 
in vertical section, is shown in Plate 3. fig. 10. In the more superficial region of the 
MDCCCLXXTII. T 
