16 THE VOYAGE OF H.M.S. CHALLENGER. 



polyps. This fact, he said, has only been proved for the Millepores : the remaining 

 Tabulata should be joined with the true polyps, with which their relations are very near 

 and intimate. The transverse partition-walls, the presence of which was held by Milne- 

 Edwards and Haime as a characteristic distinction, are structures of a very low order 

 of classincatory importance, which occur in widely different forms, and are only brought 

 about by the simultaneous emptying of the generative products from the radial 

 chambers. Where the emptying is not thus periodically simultaneous, a separate trans- 

 verse septum is formed in each of the chambers shutting off the space thus become 

 vacant. True tabulse, as he showed, are found not only in Millepora and Pocillopora, 

 but in Ccelastrcea, Alveopora, and Aster opsammia. Columnaria he considered to be 

 closely allied to Ccelastrcea, Favosites to Alveopora, Porites, &c. Heliopora being now 

 shown to be an Alcyonarian, tabulse are proved to be present in forms still more widely 

 different than is shown to be the case by Professor Verrill. The relations of Favosites 

 and Columnaria appear now in a different light. 



The opinions expressed concerning Professor Agassiz's relegation of the Tabulate and 

 Rugose Corals to the Hydroids have been various. 



Professor Allman, in his Monograph of the Gymnoblastic or Tubularian Hydroids 

 (London, jmblished for the Ray Society by Robert Hardwicke, 192 Piccadilly, 1871, 

 page 3), referred to Professor Agassiz's opinion on the subject as published in 

 his Contributions to the Natural History of the United States. He considered that 

 since we are entirely ignorant of the generative system of the Milleporida?, it was 

 much safer to wait for such verification as might be expected from further researches. 

 He hesitated to include amongst the Hydrozoal orders the Tabulate and Rugose corals. 



Count Pourtales (Illustrated Catalogue of the Museum of Comparative Anatomy at 

 Harvard College, Cambridge, Mass., No. 4, Deep Sea Corals, p. 56) placed the Mille- 

 poridse with the Hydroids. He remarked, " No observations have been made on 

 Millepora since Professor Agassiz's first announcement of the affinities of the Millepores 

 with the Hydroid polyps twenty years ago. The polyps of Millepora are very difficult 

 to observe, both because of their small size, and because they are killed by the shortest 

 contact with air ; when obtained expanded, they contract on the slightest shake of the 

 vessel containing them. I have succeeded but once, in company with Professor Agassiz, 

 in having a good view of one of the larger polyps of Millepora alcicornis. It differed 

 from the figure in the Contributions to the Natural History of the United States, vol. iii. 

 pi. xv. fig. 1, in being comparatively shorter and having larger tentacles, or rather 

 tentacular masses studded with lasso cells five in number instead of four. The mouth 

 was not seen very distinctly, but appeared to be a transverse slit in the middle of the 

 disc. It remained expanded but a short time." Pourtales rightly placed his genus 

 Plioboihrus amongst the Hydroids ; but, judging from the structure of the hard parts 

 alone, associated it with Millepora. 



