ALCYONARIAN CORALS 



119 



the Madreporarian corals, and are usually called the pseudo- 

 septa (Fig. 52). 



On making a section of a branch the pores can be seen 

 to pass down into a series of parallel tubes with imperforate 

 walls, which are divided into chambers by numerous tabulae 



(Fig. 53)- 



The corallum of Heliopora is therefore imperforate, 



tabulate, and dimorphic. 



The structure of the soft parts of Heliopora is very 

 peculiar. It might have 

 been expected from the 

 characters of the corallum 

 that the polyps would 

 prove to be dimorphic, 

 and that we should find 

 in the large pores auto- 

 zooids and in the small 

 pores siphonozooids. But 

 this is not the case. In 

 the large pores there are 

 autozooids having the 

 general characters of 

 typical Alcyonarian 

 polyps, but in the smaller 

 pores there are only 

 tubular diverticula of the 

 canal system crowded 

 with zooxanthellae and 

 showing no trace of polyp 



structures. It has been suggested that these tubular 

 cavities represent the body cavities of siphonozooids which 

 have been lost by degeneration ; but there is no evidence 

 to support that view. 



When the Hehopora is seen alive on the reef, tlie polyps 

 are usually tightly x"etracted into the larger pores, but pro- 

 jecting from the grey surface a number of small thread-like 

 worms display their active contortionate movements. These 

 worms, belonging to the Polychaet genus Leucodora, are 

 very frequently associated with Heliopora, and the thin 



l'"iG. 52. — Heliopora coerulea. A part of 

 the corallum highly magnified showing the 

 large pores with their false septa and the 

 small pores. Penetrating the surface are 

 seen five smooth cylindrical tubes of the 

 worm Leucodora. 



