1893. THE GREA T BA RRIER REEF OF A U ST R ALIA . 457 



positions in shallow parts, or when they had expanded in his tanks 

 and tubs. 



Among the Alcyonarians, the Alcyonidae, from their number and 

 abundance, are very conspicuous on the living reefs ; but as their 

 most abundant forms possess a flexible corallum with separated 

 spicules of Hme, they break up on death and add only to the general 

 powdery debris. 



Tubipora, the organ-pipe coral, is present for the most part not 

 in large patches, but scattered about further inshore than ordinary 

 corals, and in regions where there is much muddy sediment. Helio- 

 pora ccenilea was found in abundance, and while the author now 

 confirms Moseley's reference of the Heliopores to the Alcyonarians, 

 he is inclined to believe that a part of the corallum is built up by 

 commensal worms closely allied to the Lencodore ciliata, which did so 

 much to destroy the Australian oyster-fisheries. The account in the 

 book, however, would be equally consistent with a purely parasitic 

 relation of the worm to the coral. 



A new sheaf-shaped Alcyonarian, called Xenia pulsitans by the 

 author, and found in Torres Strait, is remarkable not only for the 

 size and delicate colouration of its polyps, but for certain special 

 physiological manifestations. "The expanded tentacles in this type 

 measure over an inch in diameter. The colour of the stalks and of 

 the main shafts of the tentacles is a pale beryl-green, while the 

 conspicuous tentacular pinnae, and the substance of the common 

 supporting polypary, are a pale ochreous-brown. The special physio- 

 logical phenomenon observed of this type was associated with the 

 movements of its tentacles. In all ordinary polyps, whether belong- 

 ing to the coral-secreting or skeletonless sections, the component 

 tentacles move quite independently of one another, and their action is 

 either irregularly vermiculate, or one of simple expansion and 

 retraction. In the present type, on the contrary, all the eight 

 tentacles move synchronously, opening out and contracting in a 

 continuous measured rhythm, averaging two seconds to each con- 

 traction. The action thus observed was in all respects identical 

 with the pulsating contractions of a jelly-fish, and was suggestive of 

 a less remote affinity between the Alcyonarian tribe and the medusi- 

 form Hydrozoa than subsists between the last-named tribe and 

 the coral-forming Madreporaria or skeletonless Actinozoa. This 

 suggestion of affinity receives substantial support from the fact that 

 as the radial processes throughout the jelly-fish tribe are invariably a 

 multiple of four, and most commonly eight, they thus correspond in 

 number with the tentacular organs of an Alcyonarian." 



Among the Hydrozoa the only generic type that contributes to 

 any material extent to the formation of the reef is that of Millepora. 

 The brilliant Distichopova coccinea, which resembles CoraUmm yubriim 

 except that its skeleton is commercially valueless, is found not 

 infrequently. 



