EXCAVATORS. 359 



have seen half-grown living oysters, with Cliona ex- 

 tending from the umbones, almost to the ventral 

 margin, and in one or two instances it even reaches 

 the margin. In these cases it is evident that the 

 growth of the sponge must have been more rapid 

 than that of the shell ; for the work of destruction 

 could not commence until the oyster had attained to 

 some size ; and had its growth been even equal to that 

 of the sponge, the shell ought to have reached its full 

 development, before the sponge had gained the lower 

 margin. When a shell is once attacked, the opera- 

 tions of these creatures never cease, until they have 

 extended throughout its entire substance. The 

 middle portion soon becomes almost completely 

 excavated, small pieces only remaining to divide the 

 chambers or branches. A thin plate is left on the 

 outer and inner surfaces to protect the parasite ; 

 and even these plates are ultimately riddled with 

 numerous circular holes, which are the only indication 

 of the work of destruction beneath, until some slight 

 external influence ruptures the protecting walls, or 

 the increasing growth of the tenant bursts them 

 asunder ; when the whole system of elaborately- 

 wrought chambers becoming exposed soon give way, 

 and Cliona, Sampson-like, perishes amidst the ruin 

 produced by its own energy." 



The description, which he afterwards proceeds to 

 give, of the channels made in the shells indicates one 



