358 TOILERS IN THE SEA. 



was the real agent at work in erosion. Of the exact 

 constitution of this corrosive fluid we know nothing. 

 The importance of the boring sponge in helping to 

 effect the redistribution of eternal matter, does not 

 consist in comminuting the stone into particles, but 

 in dissolving it, as sugar is dissolved in a glass of 

 water, and mingled with the sea-water in this dis- 

 solved condition. Out of this solution the innumer- 

 able shell-fish take the mineral materials, which have 

 been mingled with their blood, and from which it is 

 deposited as new layers on the shell, which, when 

 the animal dies, either is also finally re-dissolved by 

 the sponge, or falls to the bottom of the sea, as a 

 contribution to the earth's strata of future aeons." 



The most explicit, and detailed account, which we 

 possess of these burrowing sponges is that of Mr. 

 Hancock, in 1849, in which he strongly insisted on 

 the excavations being performed by the sponges 

 themselves. Although, at the time, his conclusions 

 did not meet with universal assent, his case was 

 evidently a strong one in favour of his hypothesis. 

 " On the coast of Northumberland," he says, " the sur- 

 face of almost every piece of limestone, near low- 

 water mark, is riddled by Cliona (boring sponge) ; 

 old shells, whether univalves or bivalves, are filled 

 with it ; it inhabits millepores ; and in southern 

 latitudes it buries itself in corals. Its ravages are 

 very extensive, and appear to be rapidly effected. I 



