NARRATIVE OF THE CRUISE. 
791 
and stout gastrozooid with its four tentacles, dark stomach cavity seen through the 
walls of its body and its mouth at its summit. Around arc grouped five dactylozooids, 
each with many tentacles, but without any mouth or stomach. One of the dactylozooids 
is seen bending over to feed the gastrozooid of the system. 
“ By far the most valuable discoveries, from a zoological point of view, with regard 
to the heterogeneous group which commonly goes under the name of £ corals,’ made 
during the Expedition, were those which proved that the curious Heliopora, with its 
dark blue skeleton, to be an Alcyonarian allied to the precious coral of commerce, and the 
confirmation of the late Professor Louis Agassiz’s results as to the Hydroid nature of 
Millepora. Not only were Agassiz’s conclusions amply confirmed, but another family of 
Hydroid corals was discovered in the Stylasteridse, until then believed to be allied to the 
Fig. 274. — Portion of the hard coral skeleton of 
Millepora nodosa ; twice natural size. 
Fig. 275. — System of zooids of Millepora nodosa in the expanded 
condition. Single short gastrozooid in the centre surrounded 
by five elongate dactylozooids. 
Oculinidso, the members of which exhibit in their structure even more remarkable com- 
plexity than the Milleporidse. Heliopora, the blue coral, was, until the voyage of the 
Challenger, always believed to be essentially similar in structure to ordinary Madrepo- 
rarian corals, to have its tentacles smooth and non-pinnate, and to bear them in multiples 
of the number twelve. On examination of the soft tissues, however, it was found that 
Heliopora has eight pinnate tentacles only, and that all its structure conforms through- 
out with that of other Alcyonarians. Heliopora thus proves to be the sole modern 
survivor of a large series of massive Alcyonarian corals which flourished in the Palaeozoic 
epoch, and its minute structure is of the greatest value, F now r that it is properly understood, 
as a guide to the elucidation of complications which occur in its ancestral allies. 
“ With regard to Millepora, some account lias just been given of its structure and of 
(NARR. CIIALL. EXP. — VOL. I.- — 1885 .) 100 
