BRAIN-CORALS.—STAR-CORALS. 25 
Brain-Corals (MJeandrina).*—These corals 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 
Fic. 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 (Astrez).—In the Astreas the polyps are 
very large, some having a diameter of two inches, 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. 
