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 ccenosarc. 



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 alcieornis. 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 alcieornis, 

 according to Professor Agassiz, 1 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 ccenosarc, extending in position to a considerable depth from the 

 external surface of the coral (PI. 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 alcieornis. This latter species and the Samboangan one differ from 

 the Tahitian species mainly in not having their zooids grouped in distinct systems. 



General Kemarks ox the Structure of the Milleporid.e. 



• 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 ccenosarc,. the forms of the 



1 Loc. cit., vol. iv. p. 293. 



