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 I. 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 fined 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 I. 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 Corallium ruhrwm the pinnae or 

 barbules of the tentacles are all severally introverted, as well as the tentacles themselves, 

 in Heliopora such appears not to be the case. 1 



In the atlas of the Voyage de l'Astrolabe, 2 the expanded polyps of Heliopora ccerulea 

 are figured by MM. Hombron and Jacquinot ; in figure 1 4 sixteen very short simple conical 

 tentacles are shown, in figure 1 3 only fifteen tentacles. The figures are evidently very 

 erroneous. 



In the zoology of the Voyage de l'Uranie 3 is a description of the polyps of Heliopora 

 ccerulea, 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 



1 H. de Lacaze Duthiers, lor. cit., p. 57. 



2 Hombron et Jacquinot, Voyage an Pole Sud de 1' Astrolabe et Zelle, Paris, 1842-53, Zoophytes, pi. xxix. 

 ligs. 12-14. 



3 Quoy et Gaimard, Voyage de l'Uranie, Paris, 1824. 



