25 



Brain-Corals (J/^^fc^A^jJ5^rorals form in 

 great heads twelve to fifteen feet in diameter. The polyps 

 are arranged in trenches resembling the convolutions of 

 the brain. They grow rapidly. Fig. 24 shows a head of 

 Meandrina convexa that doubled its diameter in a year 

 or grew at the rate of one inch a year under unfavorable 



FIG. 24. Meandrina convexa, Tortugas, Florida, growth of which was 

 watched by Dr. J. B. Holder. 



circumstances, being kept by the author in an aquarium, 

 or inclosure, of dead-coral rock, through which the tide 

 rose and fell. 



Star-Corals (Astrea)lv. the Astreas the polyps are 

 very large, some having a diameter of two inclies, almost 

 as large as some anemones. The tentacles are of various 

 shades green, purple, gray, and blue tints. They attain 

 a weight of several tons. 



* Meandrina spongiosa, common on the Florida reef, floats upon 

 the surface when deprived of the animal matter, and is known as 

 floating coral. 



