i66 THE SEAS 



Laccadive Islands ofi the south of India, and of far less 

 importance in others. Their chief role in reef formation 

 probably consists of forming a kind of cement which binds 

 together the masses of coral in the cracks of which collects 

 sand, formed by the breaking up of the coral and from the 

 tiny calcareous skeletons of the Foraminifera, all being 

 covered over with a firm crust of calcareous weed. An im- 

 portant centre of rock formation is frequently provided by 

 the giant clams, Tridacna (Plate 58), which grow on the 

 surface of the reefs and have shells often more than three 

 feet long and more than an inch thick. 



Sooner or later the corals die, often as a result of the 

 accumulation of silt and debris, though sometimes the 

 attacks of a filamentous seaweed and at other times ex- 

 ceptional freshening of sea water in enclosed areas, such 

 as lagoons, cause the death of great numbers of corals. 

 The corals are always bored into by a number of animals, 

 notably the Date-mussel and species of Piddock (Pholas), 

 described in the last chapter, and also by many kinds of 

 worms and sponges, but their action, by causing the corals 

 in response to strengthen their skeletons, is sometimes 

 helpful rather than the reveise. In the Red Sea and other 

 coral regions there is a fish called Pseudoscarus, which 

 appears actually to live on coral, its parrot-like beak biting 

 off pieces which are later to be found in the stomach ! 

 These fish, together with the various boring animals 

 certainly assist very materially in the breaking up of coral 

 masses after the animals w^hich made them have died and 

 the protective coating of living tissue has disappeared. 

 In this manner (and also by the eroding action of the waves) 

 a great part of the coral is broken up into sand and mud or 

 dissolved in the sea water. Some animals may play an 

 important part in this process whereby coral skeletons are 

 returned eventually whence they came, in particular the large 



