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THE POPULAR SCIENCE MONTllLY 



The reef-building corals comprise all those with a stony skeleton, 

 yet the great work is carried on mainly by a few of the principal 

 groups. Of these the following are the most conspicuous and familiar : 



The Astrazas, so called from their star-shaped cells (Fig. 6, page 261). 

 They grow in huge hemispherical masses, often twenty feet or more in 

 diameter. The brain-coral, covered with meandering furrows and ridges 

 resembling cerebral convolutions (Fig. 1, page 282). The masses are 

 large, and are shaped like the astraea. The Forties, often branched,- 



Fig. 9. 



Millipores ; Coral secreted by Jelly-fish (Aealephs). 



sometimes massive and covered with exceedingly minute cells, are rep- 

 resented in Fig. 8, page 263. Other species are branching or lamel- 

 lar, as the MilUpores (Fig. 9), which contribute largely to the ma- 

 terial of the West-India reefs. The animal, however, in this case, is not 

 a true polype, but belongs to the group of aealephs, or jelly-fishes. 

 Then there are the beautifully branched, tree-like Itfadrepores (Fig. 10, 

 page 205). Fig. 11, page 266, represents one of the most beautiful of 

 the corals produced by the aleyonoid polypes. Almost all of these are 

 flexible, and sway with the moving waters. Some kinds are too flexi- 

 ble to stand erect, and they hang from the coral ledges, as in the 

 coral caves, in gorgeous clusters of scarlet, yellow, and crimson. All 

 these corals are covered with cells, and each represents the dwelling 



