28 : THE VOYAGE OF H.M.S. CHALLENGER. 
tips of the tentacles of the zooids; but both large and small nematocysts of this kind 
occur also in the coenosare. 
The larger ovoidal nematocysts are also such as occur in Hydroids, but are not of so 
characteristic a form as the three-spined kind. They, as well as the others, were already 
figured by the late Professor Agassiz from Millepora alcicornis. These nematocyts 
never occur in the tentacles of the zooids, being confined to the ccenosarc, and being 
present in the greatest abundance in zonal masses around the bases of the zooids 
lying in the superficial layer of the ectoderm. Both these forms of nematocysts occur 
together in Gemmaria implexa, and with a similar distribution. In Millepora alcicornis, 
according to Professor Agassiz,’ these ovoid nematocysts occur in abundance in the 
tentacles of the zooids. Possibly he was mistaken in the matter. 
Both kinds of nematocyts occur, in all stages of development, in the ectoderm of the 
vascular network of the coenosarc, extending in position to a considerable depth from the 
external surface of the coral (Pl. XIV. fig. 8). 
The nematocyst 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 nematocysts. 
Most unfortunately no trace of generative organs could be detected in connection 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 dactylozooid and gastrozooid. They have both of them closely similar 
nematocysts of both forms and with a similar distribution. Moreover, the larger nemato- 
cysts have very nearly the same dimensions in all three species; they are a trifle smaller 
only in Millepora alcicornis. This latter species and the Samboangan one differ from 
the Tahitian species mainly in not having their zooids grouped in distinct systems. 
GENERAL REMARKS ON THE STRUCTURE OF THE MILLEPORIDA. 
Although, most unfortunately, no evidence as to the structure of the generative 
system of Millepora has as yet been obtained, the results here set forth yield, neverthe- 
less, I think, convincing proofs that this interesting form of coral is a true Hydroid, as 
discovered by Agassiz in 1859. The peculiar structure of the coenosarc, the forms of the 
1 Loc. cit., vol. iv. p. 293. 
