36 4 Mr. Osler on burrowing 
shell almost as soon as it has acquired sufficient thickness to 
afford them a nidus. They may, indeed, be considered to 
perform the very important office of destroying shells when 
they become useless ; thus preventing accumulations which, 
in a few ages, would fill every harbour. The thick oyster 
shells on these coasts are always occupied by a kind of sponge. 
It is a fibrous yellow pulp, filling a number of irregular 
cells, which open freely into each other, and eventually oc- 
cupy and destroy the whole shell. The cells communicate 
with the surface by a number of small, round, and polished 
holes, each of which is occupied by a tough, and apparently 
inirritable tube, which adheres firmly to its circumference. 
These holes have been observed by every conchologist, and 
were supposed by Montagu to be formed by Mya bidentata. 
The yellow substance is not peculiar to the oyster, nor is it 
disease ; fori have met with it in dead shells, and have found it 
extended to the surface through a considerable thickness of 
coralline. The penetration of all these naked animals leads 
irresistibly to the conclusion, that a shell is not essential to 
the boring process ; and it would be inconsistent with the 
simplicity observable in every part of nature, to suppose that 
she has provided such different means for accomplishing the 
same end. 
Whatever may be the instrument of penetration, it is 
situated at the lower and anterior part of the animal. Where 
the Saxicavae are numerous, their holes communicate very 
freely ; and it is common to meet with one which has attached 
its byssus to another. In this case, it is always found that 
the shell of the second has been acted on in a direction, and to 
an extent, which corresponds with the range of the foot of 
