2i8 CORALS 



apt to die at the base, leaving the bare skeletal structures 

 exposed to the attacks of various boring and otherwise 

 destructive organisms. For a time they may be protected 

 from these attacks by the overgrowth of many different 

 kinds of encrusting animal and vegetable colonies. 



Among these the most important, perhaps, are the hard 

 calcareous structures formed by the coral Algae, Litho- 

 thamnion and Lithophyllum, which form at first a thin 

 film covering the exposed parts and following its contours 

 like a crust, and then later growing beyond its support 

 to form a thallus of its own. It is often discovered, when 

 a large lump of coral is examined, that it consists of a 

 thick crust of one of these coral Algae covering a core of 

 some kind of Madrepore, as if the Madrepore had been 

 overwhelmed and smothered by the Lithothamnion. But 

 it is a question which has not been satisfactorily answered 

 whether there is really any real smothering process in the 

 production of these lumps. It seems to be most probable 

 that the encrusting Alga has simply followed the death of 

 the living tissues of its host from its base until when the 

 last polyp has died it completely surrounds and decently 

 entombs it by its further active growth. 



The coral Algae not only protect the individual Madre- 

 porarian and other more delicate corals from the onset of 

 decay, but undoubtedly play an important part in welding 

 them together to resist the action of the surf ; and on many 

 reefs where the breakers fall with great force they form, as it 

 were, an advanced post of coral reef to protect and shelter 

 the ranks of the others in the outer waters of the lagoon. 



The exposed base and stems of corals are also protected 

 by the growth of the pink discs of the Foraminifera, Poly- 

 trema, and Homotrema, by Cellepora and other Polyzoa, by 

 various kinds of encrusting Sponges, by Tunicata, and some- 

 times by masses of calcareous worm tubes. 



On the other hand, the exposed base of the coral may be 

 attacked by several species of bivalve molluscs which bore 

 great cylindrical tubes through its substance, by cirripedes, 

 worms, sponges, and even filamentous Algae, which dissolve 

 the calcium carbonate and form lesser tubes and cavities for 



