PROTECTION AGAINST INJURY. Sel 
_ would expose it, seemingly, to immediate wear from the waters 
around it, especially as the texture is usually porous. 
But nature is not without an expedient to prevent to some 
extent this catastrophe. 
In the first place, there is often a perdtheca over the 
dead corallum—that is, an outer impervious layer of carbonate 
of lime, secreted by the lower edge of the series of dying pol- 
yps, a fact in the Goniopora columna figured on page 52. 
Then, further, the dead surface becomes the resting-place of 
numberless small encrusting species of corals, besides Nulli- 
pores, Serpulas, and some Mollusks. In many instances, the 
lichen-like Nullipore grows at the same rate with the rate of 
death in the zodphyte, and keeps itself up to the very limit of 
the living part. The dead trunk of the forest becomes covered 
with lichens and fungi, or in tropical climes, with other foliage 
and flowers; so among the coral productions of the sea, there 
are forms of life which replace the dying polyp. The process 
of wear is frequently thus prevented. 
The older polyps, before death, often increase their coral se- 
cretions also within, filling the pores as the tissues occupying 
them dwindle, and thus render the corallum nearly solid; and 
this is another means by which the trees of coral growth, 
though of slender form, are increased in strength and endur- 
ance. 
The facility with which polyps repair a wound, aids in 
carrying forward the results above described. The breaking 
of a branch is no serious injury to a zoodphyte. ‘There is often 
some degree of sensibility apparent throughout a clump even 
when of considerable size, and the shock, therefore, may occa- 
sion the polyps to close. But, in an hour, or perhaps much 
less time, their tentacles will again have expanded; and such 
as were torn by the fracture will be in the process of com- 
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