PROTECTION AGAINST INJURY. 97 



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 peritheca 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 zoophyte, 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 zoophyte. 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- 



