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 classificatory 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 tabulee, as he showed, are found not only in Millepora and Pocillopora, 
but in Ce@lastrea, Alveopora, and Asteropsammia. Columnaria he considered to be 
closely allied to Celastrea, Favosites to Alveopora, Porites, &c. Heliopora being now 
shown to be an Alcyonarian, tabulz 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, published 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 Milleporid, 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 Pourtalés (Illustrated Catalogue of the Museum of Comparative Anatomy at 
Harvard College, Cambridge, Mass., No. 4, Deep Sea Corals, p. 56) placed the Mille- 
poridee 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. 
pl. xv. fig. 1, in beg 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.” Pourtalés rightly placed his genus 
Phobothrus amongst the Hydroids ; but, judging from the structure of the hard parts 
alone, associated it with Millepora. 
