48 MOLLUSCA. 
the power of forming threads of silky substance 
much stronger and more durable than those of our 
pond snails. The Common Mussel (Mytilus edulis) 
is one of these marine silk-worms; and we have a 
good many others. The bundle of threads, familiar 
to many of my readers as the beard of the shell-fish, 
is the substance in question, termed by naturalists 
byssus, a Greek word originally signifying silk ; and 
the use to which it is applhed by the animal itself 
is that of a cable to moor itself to the solid and 
immovable rock, that it may not be washed away 
by the violence of the waves. The mode in which 
the threads are formed, and the organ by which 
they are secreted, are thus described by Professor 
Rymer Jones :— 
“The foot in the Mussel is of small dimensions, 
being useless as an instrument of progression. By 
its inferior aspect it gives attachment to the horny 
threads of the byssus, which are individually about 
half an inch in length, or as long as the foot 
itself, by which, in fact, they are formed, in a 
manner quite peculiar to certain families of Con- 
chifera; no other animals presenting a secreting 
apparatus at all analogous, either in structure or 
oftice, to that with which these creatures are pro- 
vided. The manner in which the manufacture of 
the byssus is accomplished is as follows: A deep 
eroove runs along the under surface of the foot, at 
the bottom of which thin horny filaments are formed 
by an exudation of a peculiar substance, that soon 
hardens and assumes the requisite tenacity and 
firmness. While still soft, the Mussel, by means 
of its foot, applies the extremity of the filament, 
which is dilated into a kind of little sucker, to the 
foreign substance whereunto it wishes to adhere, 
