114 THE VOYAGE OF H.M.S. CHALLENGER. 
eight lobes of Heliopora just described probably occupy a similar position and have a 
similar appearance in the expanded condition of the polyps. 
Tentacles.—From the centre of the disk of lobes a tubular cavity, which may be called 
the atrium, leads down directly to the mouth (Pl. L. fig. 1). Around the mouth and 
just above it, orifices of the eight tubular introverted tentacles open into the atrium. 
The tentacles in the retracted condition are completely introverted and appear as 
tubes, the inner cavities of which would, in the expanded condition of the polyp, form 
the outer surface of the tentacle. The cavities of the introverted tentacles communicate 
directly with the atrium, as may be seen in vertical sections, by orifices which show in 
the centre a cruciform lumen (Pl. I. fig. 1, T’) formed by the folds of the ectodermal 
lining of the tentacular cavity. The retracted tentacles are directed at first horizontally 
outward from the atrium, and then turned downwards at nearly right angles to their 
former course. They rest in the intermesenterial spaces. Transverse sections of four of 
them are seen in Plate I. fig. 3. The cavities of the introverted tentacles are lined by 
a direct continuation of the ectoderm, which passes down over the inner surface of the 
atrium to enter the cavities. In their interior the ectoderm is elevated into a series of 
short stout tubercles, which no doubt project much more in the expanded condition of 
the tentacle, rendering it compound as in other Alcyonarians. In the retracted tentacles, 
as seen in Plate IL. fig. 3, three layers, outer endodermal, median connective tissue, and 
inner ectodermal, can be readily distinguished. The median probably contains muscular 
structures, but I have been unable to see them. In Coralliwm rubrum the pinne or 
barbules of the tentacles are all severally introverted, as well as the tentacles themselves, 
in Heliopora such appears not to be the case.’ 
In the atlas of the Voyage de l’Astrolabe,” the expanded polyps of Heliopora caerulea 
are figured by MM. Hombron and Jacquinot; in figure 14 sixteen very short simple conical 
tentacles are shown, in figure 13 only fifteen tentacles, The figures are evidently very 
erroneous. 
In the zoology of the Voyage de l’Uranie® is a description of the polyps of Heliopora 
cerulea, but without any figure of them, by MM. Quoy and Gaimard, the substance of 
which is as follows :—The expanded polyps have radiated tentacles and entirely hide the 
corallum when in an expanded condition. Experiments proved that the communication 
between the polyps is somewhat imperfect, since a stimulus applied to any part of 
the colony only caused the polyps in that immediate neighbourhood to contract 
themselves. 
The authors appear to have mistaken at first the expanded parasitical Leucodoras for 
* H. de Lacaze Duthiers, loc. cit., p. 57. 
* Hombron et Jacquinot, Voyage au Pole Sud de VAstrolabe et Zelle, Paris, 1842-53, Zoophytes, pl. xxix. 
figs, 12-14. 
3 Quoy et Gaimard, Voyage de ’Uranie, Paris, 1824. 
