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THE ORGAN PIPE-CORAL. 



Owing to the vast number of the animals, and the rapidity with which they increase, the 

 groups of Astraea often assume enormous dimensions ; and in the secondary and 

 tertiary rocks they frequently occur in such huge masses that whole rocks are com- 

 posed of their remains. 



In the right-hand lower corner is seen a figure of that remarkable coral which is 

 popularly called BRAIN-STONE or BRAIN-CORAL, because the convolutions into which the 

 corallium is moulded much resemble those of the human brain. 



The animals of this genus are always united together in long waved series, each 

 having a distinct mouth and series of very short tentacles. The cells are very shallow, 



Madrtpora abrotandides. 

 Aitreea pallida. 



Madrlpora cribifera, 

 EchinApora itndulata. 

 Mteandrina certbriforntis. 



and the valleys formed by their union are separated from each other by dis f inct ridges. 

 The shape of this coral alters greatly with age, somewhat resembling the top-shells 

 when young, but becoming rounded above when adult. The Brain-coral is found in 

 several of the hot seas. 



AMONG the Asteriadae, as these creatures are called, in consequence of the star-like 

 appearance of the polype or animal, the ORGAN PIPE-CORAL is perhaps the most 

 striking. It forms, as far as is yet known, the only example of the group to which it 

 belongs, and which is called Tubuliporina, on account of the multiplied series of reg- 

 ular tubes from which it is formed. Most persons are familiar with this beautiful 

 coral, with its sets of tubes arranged like the pipes of a church-organ, or the storied 

 rows of basaltic columns of the Giant's Causeway. 



