AGE OF CORAL REEFS. 177 
worn away from one part of the Reef help to 
build it up elsewhere. The Corals forming the 
Reef are not the only beings that find their home 
there: many other animals — Shells, Worms, 
Crabs, Star-Fishes, Sea-Urchins — establish them- 
selves upon it, work their way into its interstices, 
and seek a shelter in every little hole and cranny 
made by the irregularities of its surface. In the 
Zodlogical Museum at Cambridge there are some 
large fragments of Coral Reef which give one a 
good idea of the populous aspect that such a 
Reef would present$ could'we see it as it actually 
exists beneath the water. Some of these frag- 
ments consist of a succession of terraces, as it 
were, in which are many little miniature caves, 
where may still be seen, the Shells or Sea-Urchins 
which made their snug and sheltered homes in 
these recesses of the Reef. 
We must not consider the Reef as a solid, mas- 
sive structure throughout. The compact kinds of 
Corals, giving strength and solidity to the wall, 
may be compared to the larger trees in a forest, 
giving it shade and density; but beneath these 
larger trees grow all kinds of trailing vines, 
ferns, and mosses, wild-flowers, and low shrubs, 
filling the spaces between them with a thick un- 
derbrush. The Coral Reef also has its under- 
brush of the lighter, branching, more brittle 
kinds, filling its interstices, and fringing the sum- 
8* L 
