XIII ALCVONARIA — COENOTHECALIA 345 



It is found at the present day in many localities in the warm 

 shallow waters of the tropical Pacific and Indian Oceans. It 

 usually flourishes on the inside of the reef, and may form masses 

 of stone five or six feet in diameter. The coral may easily be 

 recognised, as it is the only one that exhibits a blue colour. 

 This colour usually penetrates the whole skeleton, but in some 

 forms is absent from the superficial layers. 



The skeleton consists of a number of parallel tubes with 

 imperforate walls, which are fused together in honey -comb 

 fashion. On making a vertical section through a branch of the 

 coral it is found that the tubes are divided into a series of 

 chambers by transverse partitions or " tabulae." The soft living 

 tissues of the coral, the zooids and coenosarc, are confined to the 

 terminal chambers, all the lower parts being simply dead cal- 

 careous skeleton supporting the living superficial layer. Among 

 the parallel tubes there may be found a number of larger 

 chambers that seem to have been formed by the destruction ot 

 the adjacent walls of groups of about nineteen tubes. These 

 chambers are provided with a variable number of pseudo- septa, 

 and have a remarkable resemblance to the thecae of some 

 Zoantharian corals. That Heliopora is not a Zoantharian coral 

 was first definitely proved by Moseley, who showed that each of 

 these larger chambers contains an Alcyonarian zooid with eight 

 pinnate tentacles and eight mesenteries. The zooids arise 

 from a sheet of coenosarc that covers the whole of the living 

 branches of the coral mass, and this sheet of coenosarc bears a 

 plexus of canals communicating on the one hand with the zooids, 

 and on the other with a series of blind sacs, each of which 

 occupies the cavity of one of the skeletal tubes as far down as 

 the first tabula. The zooids of Helioiwra are very rarely 

 expanded during the day-time, and it has been found very 

 difficult to get them to expand in an aquarium. The coral, 

 however, is frequently infested with a tubicolous worm allied to 

 the genus Leucodora, which freely expands and projects from the 

 surface. So constant and so numerous are these worms in some 

 localities that it has actually been suggested that Heliopora should 

 be regarded as a Polychaete worm and not as an Alcyonarian. 

 According to Mr. Stanley Gardiner, however, these worms do not 

 occur in association with the Heliopora found on the reefs of the 

 ]\Ialdive Archipelago. 



