CORALLINES. 169 



Trees, one-half of which, are animated, growing at the hottom of the 

 sea; polyps, one-half of which is imprisoned, and riveted to their 

 person ; their stomachs in the hark, their arms on a branch, their 

 movements perfect repose ! 



These minute silent workers are active and indefatigable ; their task 

 is to separate the salt and other chemical particles from the waters of 

 the ocean, and, while feeding themselves, secrete and organise the axis 

 which bears their lodging. They love the warmer regions of the ocean ; 

 in colder regions, the results of their labours are extremely limited : 

 the one forms a sward of submarine life, which carpets the rocks ; the 

 other produces animated stalactites, great shrubs, whole forests of small 

 trees. The electric cable which unites Sardinia to the Genoese fort 

 was so encrusted with corals and bryozoares, that certain portions 

 taken from the water for repairs had attained the size of a small barrel. 



The atolls present three unfailing and constant peculiarities. 

 Sometimes they constitute a great circular chain, the centre of which 

 is occupied by a deep basin, in direct communication with the exterior 

 sea, through one or many breaches of great depth. These are the 

 atolls, described more than two centuries ago by Pyrard de Laval ; 

 sometimes they surround, but at some distance, a small island, in such 

 a manner as to constitute a sort of skeleton or girdle of reefs ; finally 

 they may form the immediate edging or border of an island or continent. 

 In this last case they are called fringing littorals, or edging reefs. At 

 the distance of a few hundred yards only from the edge of some of 

 these reefs, the sea is of such a depth that the sounding-lead has failed 

 to reach the bottom. 



In order to give an idea of the general form of these atolls, 

 although they are rarely so regular, the reader is referred to PL. III., 

 which represents one of these islands of the Pomotouan Archipelago, 

 in the Indian Ocean. It represents the island of Clermont-Tonnerre, 

 figured by Captain Wilkes in the American Exploring Expedition. 

 The exterior girdle of rocks here surrounds a basin nearly circular. 

 Such is the general form the typical form, so to speak of the coral 

 isles, of which this is a fair representation. 



The zoophytes which form these mineral accumulations belong to 

 diverse groups, and nowhere have the results of observations made 

 upon these atolls been more minutely described than in Mr. Darwin's 



