30 THE VOYAGE OF H.M.S. CHALLENGED . 



as already explained, adopted a distinctive term for the calcareous skeleton of the 

 Hydrocorallinae. In Alcyonium 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 Cerianihus. 

 The close resemblance in the histological structure of the calcareous skeleton formed 

 by animals so different as Alcyonaria (Heliopora ccerulea), 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 Porosphcera (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 Milleporidse, on which Mr Carter 1 

 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 2 

 is opposed to Mr Carter's conclusions, to which, nevertheless, I am, from the evidence 

 adduced, inclined to adhere. Mr Carter 3 has described a species of Millepora, M. wood- 

 wwrdi, as occurring in the lower chalk. Apparently no older representative of the genus 

 is known. 



Vegetable Parasites of the Milleporid.e. 



In my paper On the Structure of Heliopora ccerulea (Phil. Trans. Eoy. 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, 4 who summarises the results obtained 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 hoc. tit. Also, On Stromatopora, Ann. and Mag. Nat. Hist., vol. ii. p. 85, 5 ser., 1878. On the probable nature of 

 the animals which produced the Stromatoporidse traced through Hydractinia, Millepora alcicornis, and Ghcmnopora 

 to Stromatopora, ibid., vol. ii. p. 304, 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 Hydractinia; and on the identity in structure of Millepora alcicornis and Stromatopora, Ann. 

 and Mag. Nat. Hist., vol. i., 5 ser., 1878. 



4 Professor P. Martin Duncan, F.E.S., On some Thallophytes parasitic within recent Madreporaria, Proa Roy 

 Soc, No. 174, 1876, p. 238 ; On some Unicellular Algse parasitic within Silurian and Tertiary Corals, &c, Quart. Journ. 

 Geol. Soc, May 1876, p. 205. 



