CORALS 147 
the species so much used for ornamental purposes, has an exceed- 
ingly hard and stony character. The madrepore corals are called 
reef-builders, but not in the sense of constructors of reefs. They 
do not erect definite structures as bees do. It is the aggregation 
of the skeletons of the dead polyps, together with other agencies, 
which forms a reef. The coral polyps, though so minute individ- 
ually, are almost infinite in numbers. It is estimated that the 
colonies rise one half of an inch in ten years. 
Corals live at different depths in the sea. Those which form 
the base of the reef are the astreeans, which do not live above a 
depth of six fathoms. They are characterized by little star-shaped 
spots on the corallum, the radial partitions meeting in the center 
of the spots, or corallites. Next above the astreans come the 
meandrinas and the porites. The former have elongated openings 
which extend in waving furrows over the surface. The porites 
resemble astreeans, but the pits are smaller, with fewer partitions, 
and the substance is more porous. Above these and capping 
the reefs are the beautiful branching and palmate madreporians, 
together with millepores, numerous varieties of’ sea-fans (Gor- 
gonacea), and the calcareous seaweeds (nullipores), making a 
garden of beautiful branching forms of every shape and color. 
The coral reef is as thickly inhabited by other living organisms 
as is the forest by birds and insects. Mollusks, worms, crabs, 
starfishes, and sea-urchins find resting-places there and work 
destruction to the coral masses, as they bore and penetrate the 
reef in various ways until large fragments of it are detached and 
either washed by the waves to places far from their foundation, 
or ground to sand, which, filling the interstices of the reef, adds 
to its solidity. 
The living coral is quite different in appearance from the 
bleached skeletons commonly seen. The surface of the corallum 
is often soft and downy, from the numerous waving tentacles, 
and its coloring is vivid and varied. The madrepores are pink, 
yellow, green, brown, and purple. Twubipora, the organ-pipe coral, 
has green polyps emanating from its red tubes. White polyps in 
star-like form dot the branches ‘of the red coral of commerce, 
Corallina rubrum. The whole mass of Helipora is bright blue, 
