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



the surface of the ccenosteum in well-marked and separated systems, and in this respect 

 is more closely allied to Millepora plicata, Millepora foliata, and Millepora ehrenbergi, 

 as described by MM. Milne-Edwards and Haime. 



In the fresh condition the growing tips of the lobes of the coral are of a bright 

 gamboge-yellow colour, which shades off into a yellowish-brown on the sides and bases 

 of the lobes. The expanded zooids have the appearance of a close-set pearly white 

 down upon the surface of the ccenosteum. 



The zooids seem to be somewhat capricious in the matter of expansion. I made one 

 attempt to obtain them expanded, in which I carefully cut off masses of the coral by 

 means of a chisel and transferred these to a glass vessel without lifting them above the 

 surface of the water. The zooids did not expand. Mr Murray succeeded on both 

 occasions on which he collected specimens, although he exposed his to the air for a 

 moment on transferring them to a vessel. Dana, Pourtales, and others speak of the 

 peculiar difficulties attending the observation of the animals of Millepora. The ccenosteum 

 is so hard that it is almost impossible to break off a small flake without damaging 

 the soft structures to such an extent that the animals fail to expand. The animals, 

 as expanded on larger masses of the ccenosteum, can only be examined with very low 

 powers of the microscope, which, on account of the very small size of the animals, are 

 unable to show sufficient details in their structure. A momentary view of one dactylo- 

 zooid was, however, obtained under Hartnack's objective No. 4, eyepiece No. 3. I 

 obtained the view of the expanded zooids only on the morning on which H.M.S. 

 Challenger was steaming out of Papiete Harbour. The animals remained expanded 

 about two hours, but the motion of the ship interfered considerably with the investiga- 

 tion of them. 



Literature of the Subject. 



M. Milne-Edwards (Hist. Nat. des Coralliaires, Paris, 1860, torn. iii. p. 224) formed 

 the family Milleporidae to include a series of genera, amongst which Millepora and 

 Heliopora are the only recent ones, whilst he classed the Pocilloporinse as a sub-family 

 of the Favositidse, together with the Milleporidae and Seriatoporidge, under his section of 

 the Madreporaria Tabulata. The section was characterised by him as having the corallum 

 composed essentially of a much developed mural system, with the visceral chambers 

 divided into a series of stories by complete diaphragms or transverse floors, the septal 

 arrangements being rudimentary or represented by trabecular, which have a greater or less 

 extension in the mtertabulate spaces. In his description of the Milleporidee (I. c.) M. 

 Milne-Edwards referred to Professor Agassiz's then recent paper, entitled "Les Animaux des 

 Millepores sont des Acalephes Hydroides et non des Polypes" (Bibl. Univ. de Geneve, 

 Arch, des Sci., Mai 1859, torn. v. p. 80), to the following effect: — "At the moment of 



