304 THE INHABITANTS OF THE SEA. 
which, sadly irritating the nerves of the poor creature, generaliy 
bring$ it to the surface. He must, however, be very quick in 
grasping it firmly, for should he fail, the animal speedily sinks 
again into the sand and will remain there, being either in- 
sensible to the additional irritation or its instinct of self- 
preservation teaching it to remain beneath. 
The pholades, which have very delicate milk-white valves, 
burrow holes in limestone or sandstone rocks, though occa- 
sionally they content themselves with houses of clay or turf. 
How creatures invested with shells as thin as paper and as 
brittle as glass are able to work their way throngh hard stone 
has long been a puzzle to naturalists, some of whom asserted 
that they attained their object by means of an acid solvent, 
others that they bored like an auger by revolving; but recent 
investigations have discovered that their short and truncated 
foot is the chief instrument they use in their mining operations, 
being provided at its base with a rough layer of sharp erystals 
of flint, which, when worn off, are soon replaced by others, and 
act as excellent files. 
In several of the sedentary genera the rudi- 
mentary foot, thougb incapable of locomotion, makes 
itself useful by spinning a bundle of silken threads, 
called byssus, or beard, which serve to anchor the 
animal to any solid submarine object as firmly as a 
ship in harbour. Generally the connection is per- 
manent, but some species, among others the edible 
mussel, are able to detach the filaments from the 
glandular pedicle situated at the inferior base of the 
foot which originally secreted them, and then to seek 
another point of attachment. 
If the byssus be examined under a powerful lens, 
before any of the filaments are torn, it is easy to per- 
ceive that these are fixed to submarine bodies by 
means of a small disc-like expansion of their ex- 
tremities of various extent, according to the genus 
poclensor and species. Certain genera are celebrated for the 
a. Foo. abundance and fineness of their byssus; that of the 
Pinne, or Wing-Shells, among others, which are very common 
in some parts of the Mediterranean, and attain a considerable 
size, is so long and firm that in Naples it is sometimes manu- 
