MOLLUSCA. 49 
and fastens it securely. Having accomplished this 
the foot is retracted; and the thread, of course, 
being drawn out of the furrow where it was secreted, 
is added to the bundle of byssus previously ex- 
isting, all of which owed its origin to a similar 
process.” * 
Whoever has attempted to wrench up a Mussel 
from one of those shallow rock-pools, in which they 
lie as closely packed as paving stones, will have 
had proof of the great strength of these threads, no 
small violence being required to detach one. But 
there is an example on record, where the strength 
of the threads has been turned to such account 
as to give this Mollusk a second claim to be in- 
cluded in the list of such species as are beneficial 
to man.—‘‘ At the town of Bideford, in Devonshire, 
there is a long bridge of twenty-four arches across 
the Torridge river, near its junction with the 
Taw. At this bridge the tide flows so rapidly that 
it cannot be kept in repair by mortar. The Cor- 
poration, therefore, keep boats in employ to bring 
mussels to it, and the interstices of the bridge are 
filled by hand with these mussels. It is supported 
from being driven away by the tide entirely by the 
strong threads these mussels fix to the stonework ; 
and by an act, or grant, it is a crime liable to 
transportation for any person to remove these 
mussels, unless in the presence and by the consent 
of the corporative trustees.” 
There are bivalve shells allied to the mussel, 
called Penna, usually of very large size, but of thin 
and delicate structure. ‘The threads spun by these 
are long, fine, glossy, and produced in great abun- 
dance; they are capable of being twisted like silk, 
. * Animal Kingdom, p. 383. 
E 
