770 MOLLUSC A~-PHOLAS. 
in the body of the hardest marble. In their proper shell they assume diffe 
rent figures; but, in general, they somewhat resemble a muscle, except that 
their shell is found actually composed of five or more pieces, the smaller 
valves serving to close up the openings left by the irregular meeung of the 
two principal shells. But their penetration into rocks, and their residence 
there, makes up the most wonderful part of their history. 
This animal, when divested of its shell, resembles a roundish soft pud- 
ding, with no instrument that seems in the least fitted for boring into stones, 
or even penetrating the softest substance. It is furnished with two teeth 
indeed; but these are placed in such a situation, as to be incapable-of touch- 
ing the hollow surface of its stony dwelling; it has also two covers to its 
shell, that open and shut at either end; but these are totally unserviceable 
to itasaminer. The instrument with which it performs all its operations, 
and buries itself in the hardest rocks, is onlv a broad fleshy substance, some- 
what resembling a tongue, that 1s seen issuing Irom tne pottom otf its sme. 
With this soft, yielding instrument, it perforates the most solid marble; 
and having, while yet little and young, made its way, by a very narrow 
entrance into the substance of the stone, it then begins to grow bigger, and 
thus to enlarge its apartment. 
When it has buried its body in a stone, it there continues for life at its 
ease; the sea-water that enters at the little aperture supplying it with luxu- 
rious plenty. When the animal has taken too great a quantity of water, it 
is seen to spurt it out of its hole with some violence. Upon this seemingly 
thin diet, it quickly grows larger, and soon finds itself under a necessity 
of enlarging its habitation and its shell. The motion of the pholas is slow 
beyond conception ; its progress keeps pace with the growth of its body; 
and in proportion as it becomes larger, it makes its way farther into the 
rock. When it has got a certain way in, it then turns from its former di- 
rection, and hollows downward; till at last, when its habitation is completed, 
the whole apartment resembles the bowl of a tobacco pipe; the hole in the 
shank being that by which the animal entered. 
But they are not only supplied with their rocky habitation ; they have also 
a shell to protect them; this shell grows upon them in the body of the rock, 
and seems a very unnecessary addition to that defence which they have 
procured themselves by art. These shells take different forms, and are often 
composed of a different number of valves; sometimes six; sometimes but 
three; sometimes the shell resembles a tube with holes at either end, one 
for the mouth, and the other for voiding the excrements. ; 
