STKUCTURE OF A SPECIES OE MILLEPOEA. 
131 
ment of the tentacles, and in the nature of the thread-cells*, Millepora seems to 
resemble such a form as Gemmaria implexa. The real affinities of Millepora amongst 
the Hydroids cannot, however, be determined until the mode of reproduction is dis- 
covered. 
It is a remarkable fact that the corallum of Millepora seems undoubtedly to be gene- 
rated by the ectoderm. In Anthozoa this is not the case, the corallum being developed 
from a mesoderm, as appears certain in the latest accounts of the matter from M. 
LACAZE-DuTHiERs’sf researches on Astroides calycularis, and from those of KowalewskyJ 
on Astrcea and on Alcyonium digitatum. In Alcyonium two elements are recognized by 
Kowalewset as composing in the embryo the “intermediate layer” (mesoderm), viz. 
a homogeneous membrana propria, which lies internally and penetrates the mesenterial 
folds, and a peculiar thin layer of cells, which lies externally to this membrana propria. 
It is from this thin layer of cells that the gelatinous connective tissue, the spicules, and 
canal networks are formed. This special layer does not exist in other corals nor in 
Cerianthus. 
The close resemblance in the histological structure of the coralla formed by animals 
so different as Alcyonaria ( Heliopora ccerulea ), Zoantharia, and Hydroida is a remarkable 
fact. The corallum of Millepora is distinguished from all other coralla by the presence 
within it of systems of canals branching in an arborescent manner. The presence of 
such structures in fossil corals will be sufficient evidence of the affinities of such corals 
with Millepora and the Hydroida. Now that Heliopora has been shown to be an 
Alcyonarian, there remains only one living genus, Pliobotlirus (Pourtales), to form with 
Millepora the family Milleporidte. Of Pliobotlirus the soft structures have not been 
examined. 
No representative of the genus Millepora appears to be known as existing in the fossil 
condition, at least none such is mentioned in Quenstedt’s 4 Petrefactenkunde ’ or by 
M. Milne-Edwards. A careful study of the internal structure of the various extinct 
corals which have been associated with Heliopora and Millepora amongst the Tabulata 
would show which of them have real affinities with Millepora. It would be well if the 
term Tabulata were dropped altogether, since it has reference to a structure common to 
certain Alcyonaria, Zoantharia, and Hydroida, and being not characteristic of any 
natural group only tends to confusion. 
HALS. ‘ Challenger,’ S.E. Pacific, 
24th December, 1875. 
* Note . — It would seem that a classification and nomenclature of the various forms of thread-cells is much 
needed, since these forms appear to he of classificatory value in the Coelenterata. Certain forms are peculiar 
to Hydroids, e.g., others to Alcyonaria. 
t Lacaze-Dxtthieks, H. de. “ Developpement des polypes et de leur polypier,” Comptes Eendus, 1873, 
t. lxxvii. (Hoffman und Schwalbe, Jahresberichte, 1875.) 
+ Kowalewset, A. “ Untersuchungen fiber die Entwicklung der Ccelenteraten,” Haehrichten der kaiserlichen 
Gesellschaft der Freunde der Haturerkenntniss, der Anthropologie und Ethnographic. Moskau, 1873. ( Ibid .) 
