CORAL REEFS 167 



sea cucumbers most of which live on the surface of the coral, 

 where they swallow coral sand and fragments, reducing it to 

 mud within their bodies, while one kind is said to burrow 

 like an earthworm (whose habits and importance it so 

 closely imitates) swallowing the coral rock and throwing 

 up great casts of pulverised limestone. These animals are 

 of many kinds, sizes and colours, attaining to lengths of over 

 two feet and corresponding thicknesses. They are so 

 numerous on the Great Barrier Reef of Australia as to 

 provide the material for an important fishery, their dried 

 bodies, known as biche-de-mer, being exported to China, 

 where they are esteemed as a delicacy. The importance of 

 these, and similar, creatures in the conversion of coral 

 limestone into sand and mud is great. 



A reef of living coral (Plate 62) is literally a sea garden. 

 The corals themselves are of all colours, many being brown 

 with violet, pink or white polyps, others scarlet, green or 

 yellow; the polyps of some corals are foliaceous with long 

 tentacles while others are velvety masses with tentacles 

 almost absent. And the other animals have colours just as 

 vivid. There are many fish of the most varied and brilliant 

 hues which dart hither and thither in little shoals amongst 

 the trees of the coral forest, there are the many-coloured sea 

 cucumbers, large tube-worms with brilliantly coloured 

 crowns of tentacles, large sea anemones, innumerable crabs 

 and all manner of other crustaceans, many of which live in 

 definite association with the corals, a like variety of molluscs 

 and all manner of encrusting organisms such as sea mats 

 and sponges. 



Reef-building corals can only flourish in shallow water 

 within the tropical zone, being seldom in abundance below 

 about thirty fathoms and needing a surface temperature 

 of at least seventy degrees Fahrenheit, so that they are 

 confined to a belt extending between thirty degrees north 



