40 CORALS 



two species wiiicli are recognised to be highly variable in 

 form. 



More interesting than the settlement of this difficult 

 and highly controversial species question, however, would 

 be the discovery of the determining cause of the association 

 of the worm and the coral. It seems highly probable that 

 if the coral larva settles down on a little shell that has no 

 worm in it, it could not long survive, for it would soon be 

 rolled over and smothered in the sand either by the action 

 of the currents of water or by any fish or crab that passed by. 

 Does the coral larva, therefore, select shells already 

 inhabited by a worm or does it simply trust to chance ? 

 In the latter case, thousands must die for every one that 

 survives. It may be, however, that there is a kind of 

 unconscious selection, the larva finally settling down onlv 

 on a shell whose stability — due to the presence of the worm 

 — has been tested. 



The Heterocyathus is not the only coral associated with 

 this or a closely related worm. The same thing is found in 

 the perforate Eupsammiid coral Heteropsammia, occurring 

 in the same seas and under similar conditions. 



Desmophyllum. — The largest of the Turbinoliid corals 

 is Desmophyllum crista-galli, which attains to a height of 

 4 or 5 inches and a diameter of li inches at the margin of 

 the calyx. It has been found in deep water in the Atlantic 

 slopes off the British coasts and in other localities. A giant 

 specimen of this species, 5^ inches in height, was found by 

 the Challenger expedition in 345 fathoms of water off the 

 coast of Patagonia. The living polyp of this genus has 

 been studied and drawn by de Lacaze-Duthiers. 



Flabellum. — The widely distributed, and in some 

 localities very abundant, genus Flabellum is usually placed 

 in a separate subdivision of the family on account of the 

 peculiar formation of the wall of the calyx. 



Most of the specimens belonging to the genus are not 

 round, but oval in section, being, as it would seem, laterall\- 

 compressed. The longest of the diameters is in the plane 

 of the directive septa. When viewed from the side, Fla- 

 bellum has the shape of a fan with the handle sharply 



