38 THE SHELL-FISH OF THE COAST. 
a process which has not yet been clearly made out. 
The animal is provided with a double respiratory 
siphon, which can be extended to a length consid- 
erably in excess of that of the shell itself. When 
placed in a dish of sea-water, the animal, after it 
has fully recovered from its consciousness of danger, 
slowly begins to thrust out this respiratory appa- 
ratus, whose action can now be clearly followed. 
If there should happen to be minute particles of 
foreign matter in the water, it will be observed that 
these are attracted in the direction of the lower 
division of the siphon, while they are just as posi- 
tively repelled from the upper. This indicates that 
a current of water sets in through the lower or ‘in- 
current’ orifice, and that a similar current passes out 
at the same time through the upper or ‘ excurrent’ 
orifice. A perpetual circulation is thus kept up 
about the body of the animal, feeding the gills, and 
taking with it a certain amount of nutriment neces- 
sary to the existence of the animal. This arrange- 
ment is found in all the siphonated bivalves. 
Like other shell-fish that bury themselves in the 
sand or mud, the Petricola doubtless seeks its se- 
clusion from motives of self-protection. By many 
it has been supposed that the boring is accomplished 
by a peculiar revolution of the shell, during which 
the serrated ridges on the surface would act like a 
rasp, wearing and tearing as the work of excavation 
progressed. That this is not likely to be the true 
method of operation is indicated by the fact that 
there are a number of true stone-borers, which have 
an ornamentation almost identical with that cover- 
