552 DR. F. w. JONES UN GROWTH-FORMS [June 18, 



is really wonderful. A pool in wliicli numerous fiourishing 

 colonies live, quite on a sudden may show the advent of this 

 alga, and every portion of every colony which may receive a. 

 chance injury at once becomes the site of the growth of these fine 

 threads. I believe that it is always at the site of injury that the 

 attack of the alga starts, but its growth soon invades, and invari- 

 ably kills, the living healthy portions of the colony. A colony 

 once faii'ly invaded by this parasite rapidly dies, and yet it never 

 succeeds in obliterating coral-growth, for as suddenly as it came 

 to the barrier pools, it goes. Spring tides and hot weather seem 

 to promote its growth, or perhaps lower the resistance of the 

 coral colonies, for when tlie rock-pools are left long to swelter in 

 the sun, with but little depth of water in them, then the alga 

 seems to be most active. It is a great factor in causing the death 

 of the atoll corals, and ranks in Cocos far in advance of the boring 

 sponges, worms, or molluscs as a destructive agent. 



The more obvious boring creatures do not cause damage to the 

 colony other than to weaken its structure, and lead to its more 

 ready destruction by the action of the waves or moving fragments ; 

 and beyond this, they cannot be rightly considered as effective 

 enemies of coral-growth. There seems to be indeed an almost 

 symbiotic relation between certain boring animals and the corals 

 that they have chosen as their hosts, for coral -growth extends and 

 strengthens their tubes by sympathetic growth, and the cavities 

 of the molluscs in many cases expand the living area of the surface 

 of corals by causing irritation and repair. It is an extraordinary 

 thing to see the extent to which a colony of Millepora complcmata 

 may be riddled with the wide smooth-walled tubes, and yet not 

 be appreciably weakened, and the result of observation on such 

 colonies is that the borings are very little harmful to the colony. 



Another cause of coral death to which much importance has 

 been attached is the exposure caused by the receding tide, for it 

 has been said that corals cannot survive even a temporary exposure 

 to the sun and air. 



iSince Darwin first claimed this as an axiom of coral bionomics, 

 a great deal has been taken for granted with regard to the effects 

 of exposure, and yet every fresh investigator has attached less and 

 less importance to it. Now as a matter of fact there is no species 

 of coral in this atoll that is not able to withstand an exposure of 

 many hours to the mid-day sun, with from 6 inches to a foot of its 

 apical growth above the water : there is no barrier species that 

 does not normally suffer this at mid-day spring tides. There are 

 many isolated rocks that are ordinarily exposed for two feet at low 

 tide, on which living corals flourish luxuriantly. When season 

 and winds combine to cause tides abnormally low, it is possible to 

 go from island to island along the barrier-flats, and for the greater 

 part of the journey to walk in but a few inches of water ; and if 

 such a walk be taken during a low tide at hot mid-day, then the 

 smell of the exposed coral is almost overpowering and may be 

 noticed far out in the lagoon. Coral has an odour that is peculiarly 



