STBUCTUBE OF A SPECIES OF MILLEPOEA. 
129 
larger ones. They form the bulk of the spheroidal tips of the tentacles of the zooids; 
but both large and small thread-cells of this kind occur also in the hydrophyton. The 
larger ovoidal thread-cells are also such as occur in hydroids, but are not of so charac- 
teristic a form as the three-spined kind. They have been already figured by the late 
Professor Agassiz from Millepora alcicornis. These thread-cells never occur in the ten- 
tacles of the zooids, being confined to the hydrophyton, and being present in the 
greatest abundance in zonal masses around the bases of the zooids lying in the super- 
ficial layer of the ectoderm. Both these forms of thread-cells occur together in 
Gemmaria implexa, and with a similar distribution. 
Both kinds of thread-cells occur, in all stages of development, in the ectoderm of the 
vascular network of the hydrophyton, extending in position to a considerable depth from 
the external surface of the coral (Plate 3. fig. 16). 
The thread-cell appears to be developed out of the nucleus of the ectodermal cell, the 
ectodermal cell becoming much enlarged and forming a wide chamber, in which the 
process of development takes place. The ovoid nucleus becomes enlarged together with 
the cell, but not at all in the same proportion, the cell always appearing as a wide 
cavity around it. The nucleus, as it enlarges, has a rounded nucleolus developed at one 
end of it. The nucleolus has large granules developed within it, whilst the nucleus 
becomes finely granular. In the next stage one large coil of the thread appears in 
the nucleus. Nothing further could be made out from the hardened specimens as to 
the development of the thread-cells. 
Most unfortunately no trace of generative organs could be detected in connexion with 
any of the zooids, neither in the Millepora from Tahiti nor in the other two species 
examined. These other two species have essentially the same structure as the Tahitian 
species, having mouthed and mouthless zooids. They have both of them closely similar 
thread-cells of both forms and with a similar distribution. Moreover the larger thread- 
cells have very nearly the same dimensions in all three species ; they are a trifle smaller 
only in Millepora alcicornis. This latter species and the Zamboangan one differ from the 
Tahitian species mainly in not having their zooids grouped in distinct systems. 
Vegetable Parasites. 
I have already, in my paper on Heliopora ccerulea, referred to the vegetable organisms 
infesting the corallum of Millepora. I have here only to add that the mycelium and 
spores are not confined to the calcareous structures, but occur also in abundance 
amongst the soft superficial tissues ; and it appears probable that they become included 
within the calcareous tissue by the calcareous matter being deposited around them as 
the corallum is extended by growth. They are extremely abundant in the Tahitian 
species, and have a decided green colour*. 
* These organisms have, since this paper was written, been made the subject of two memoirs by Prof. P. 
Martin' Duncan, F.E.S. See “On some Thallojohytes parasitic within recent Madreporaria,” Proc. Eoy. Soc. 
no. 174, 1876, p. 288 ; “ On some Unicellular Algae parasitic within Silurian and Tertiary Corals, &c.,” Quart. 
Joum. Geol. Soc., May 1876, p. 205. 
