THE NERITIC ZONE— LIFE IN SHALLOW WATER 



441 



reefs are built up from the dead skeletons of a bewildering variety 

 of Corals, simple or colonial, and the animals to which they belong 

 are closely related to the sea-anemones. Some Corals live on 

 the floor of the deep sea, 

 but the reef-builders, so 

 far as we know, cannot 

 exist in water deeper than 

 about 40 fathoms. Since 

 some reefs extend down- 

 wards into much greater 

 depths (their foundations 

 consisting of the skeletons 

 of dead polypes, fig. 1293), 

 Darwin came to the con- 

 clusion that such reefs had 

 been formed in areas where 

 the sea-floor was sinking, 

 but at so slow a rate that 

 upward growth kept pace with it. The theory affords a simple 

 explanation of the ring-shaped reefs known as atolls, which might 

 be supposed to have come into existence from the gradual sink- 

 ing of islands fringed by reefs (figs. 1294, 1295). Borings recently 

 made on coral islands lend strong support to the hypothesis. 



Fig. 1294. — An encircling Coral-Reef in Plan and Section. By- 

 gradual sinking of the island, with corresponding upgrowth of coral, 

 an atoll (fig. 1295) might be formed. 



An Atoll 



The population of the Neritic zone further includes large 

 numbers of hydroid zoophytes and jelly-fishes {Hydrozoa), these 

 being in some cases the fixed and free-swimming stages in the 

 life -history of the same , species. 



