698 Marvels of the Universe 
bivalve mollusc has a pair of these siphons, one admitting fresh supplies of water to the gills, the 
other carrying it off. 
Sometimes these siphons are united to form one single tube, and certain Rock-borers have 
enclosed them in an outer tube or sheath of a horny substance, such as composes the hinge of 
Photo bu [Bernard C. White. 
BORER SHELLS. 
The topmost shell is that of the Streaked Martesia, showing the horny tube 
which envelops the upper parts of the shell and serves as a protection for the 
extended siphons of the snail. The next is the Coral-eater; the third one of the 
Chalk-borers, and the lowest is the Date Mussel. Notice the similarity in shape 
and the file-like ridges of three of the shells. (Natural size.) 
the two valves of the shell. The 
Streaked Martesia, shown on this 
page, 1s an instance of such a 
provision. The horny tube lines 
the whole length of the passage 
which it has bored and merges 
into the hinder end of the 
shell. In the Paper Piddock, 
this tubing has only been 
partially developed and forms 
a horny cup surrounding the 
siphons. 
The chief of the Borers is 
the Common Piddock, which is 
shown in its rocky burrow in 
the illustration on page 697. Its 
shell is a remarkable example 
of Nature’s architecture, for it 
has been planned, and its fabric 
has been strengthened, to bear 
the extra strain of the engineer- 
ing work practised by its in- 
habitant. It possesses four 
shields over and above the two 
regular shells. Two of these 
are placed side by side at the 
hinge, like the buckles of a 
belt, and a third, which is con- 
siderably stouter, locks them 
together from behind; — the 
fourth, which is shaped some- 
thing like the pen of a squid, 
runs upward along the division 
from the hinge shield to the 
rounded extremities of the 
shell and protects their upper 
margins. 
The whole of the pressure 
of the Piddock as it lies in its 
hollowed chamber rests upon 
the beak or foremost margin of 
the shells. Now, the Piddock 
uses his shell as a rasp to work 
away the rock, and a good deal 
of pressure is necessarily brought 
