236 OYSTERS, AND ALL 'ABOUT THEM. 



I 



cliona ; old shells, whether univalves or bivalves, are filled 

 with it. "I have seen," says that intelligent observer, 

 " half- grown living oysters with cliona extending from the 

 umbones almost to the ventral margin, and in one or two 

 instances it even reaches that 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. 



Only let this creature commence its attack, and it 

 pauses not until it has acted throughout the entire sub- 

 stance. Before long the middle portion is almost com- 

 pletely excavated, and only small pieces remain to divide 

 the chambers that have been formed. As, however, in the 

 working of Thames Tunnel, there was "a shield' for the 

 defence of the miners, so the parasite leaves a thin plate 

 on the outer and inner surfaces for its protection. 



Yet this serves only for a time ; numerous circular 

 holes are ultimately drilled in these plates, which are the 

 only indication of the work of destruction beneath, until 

 some slight force from without ruptures the protecting 

 walls, or the increasing growth of the tenant bursts them 

 asunder. The whole system of elaborately wrought cham- 

 bers, becoming exposed, soon gives way, and the cliona, 

 Samson-like, perishes amidst the ruin produced by its own 

 energy. 



The question naturally arises, How can such cavities 

 be produced ? Evidently by some instrument with which 

 it is provided, like the whelk, which with its proboscis can 

 bore holes in shells, or by a solvent. On this point Mr. 



