30 THE VOYAGE OF H.M.S. CHALLENGER. 
as already explained, adopted a distinctive term for the calcareous skeleton of the 
Hydrocoralline. In Aleyonium two elements are recognised by Kowalewsky as com- 
posing 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 calcareous skeleton formed 
by animals so different as Aleyonaria (Heliopora cerulea), Zoantharia, and Hydroida is a 
remarkable fact. The whole of the Milleporidee at present known appear to be naturally 
referable to the one genus Millepora, unless Porosphera (Steinm.), a Cretaceous fossil 
is, as suggested by Alleyne Nicholson, a Milleporid. I am unable to offer an opinion as 
to the alliance of Stromatopora and its congeners to the Milleporide, on which Mr Carter" 
insists, since I have as yet had no opportunity of studying the structure of these fossils. 
If Stromatopora is a Milleporid, the family dates back to Silurian times. Dr Dawson” 
is opposed ‘to Mr Carter’s conclusions, to which, nevertheless, I am, from the evidence 
adduced, inclined to adhere. Mr Carter® has described a species of Millepora, M. wood- 
ward, as occurring in the lower chalk. Apparently no older representative of the genus 
is known. 
VEGETABLE PARASITES OF THE MILLEPORID®. 
In my paper On the Structure of Heliopora cerulea (Phil. Trans. Roy. Soc., vol. Lxvi. 
part 1, p. 116) I described certain vegetable parasitic organisms as found in the tissues 
of Millepora and Pocillopora. These organisms have been made the subject of memoirs 
by Professor Martin Duncan,* who summarises the results obtamed by Leuckart, the 
original discoverer of these parasites in 1851, and subsequent observers, such as Dr 
Carpenter (Bowerbank), Wedl, and Kolliker. The parasites are of essential interest since 
they occur in deep-sea corals, and are, as far as is yet known, the only vegetable 
organisms occurring at great depths. Professor Duncan refers them to the genus Achlya 
(Saprolegnia). Both a species of Millepora obtained at Samboangan, in the Philippine 
Islands, and the Millepora nodosa of Tahiti were found to be infested by these parasites. 
1 Loc. cit. Also, On Stromatopora, Ann. and Mag. Nat. Hist., vol. ii. p. 85, 5ser., 1878. On the probable nature of 
the animals which produced the Stromatoporide traced through Hydractinia, Millepora alcicornis, and Chaunopora 
to Stromatopora, ibid., vol. ii. p. 804, 5 ser. On the Mode of Growth of Stromatopora, ibid., vol. iv. p. 101, 5 ser., 
1879. On the Structure of Stromatopora, vol. iv. p. 353, 5 ser. 
2 Stromatopora as distinguished from Millepora, Ann. and Mag. Nat. Hist., vol. xix., 4 ser., 1877. 
3 On new species of Hydractinize and on the identity in structure of Millepora aleicornis and Stromatopora, Aun. 
and Mag. Nat. Hist., vol. i., 5 ser., 1878. : 
4 Professor P. Martin Duncan, F.R.S., On some Thallophytes parasitic within recent Madreporaria, Proc. Roy, 
Soc., No. 174, 1876, p. 238 ; On some Unicellular Alge parasitic within Silurian and Tertiary Corals, &c., Quart. Journ. 
Geol. Soc., May 1876, p. 205. 
