Sometimes, however, mussels do a great deal of harm, for they will get into an oyster-bed and fasten themselves down upon the shells of the oysters. Their byssus threads then form a kind of thick mat, which collects and holds the mud that is brought up by the tide every time that it rises; and this very soon covers the oysters entirely up, and smothers them to death.
Mussels do not remain fastened down in one place for the whole of their lives, however, as oysters do. They can crawl about quite easily whenever they like. And they do this, also, by means of their byssus threads. First they move a few of these threads forward, and take a fresh hold with them; then they draw the rest up after them; and then they move the front ones forward once more, and so on over and over again.
Mussels are very largely used for food, and also as bait for deep-sea fishing. In the Firth of Forth alone, indeed, nearly forty millions of these creatures are collected every year for this latter purpose alone, or one for every man, woman, and child in England and Scotland and Wales!
PLATE XIV
THE HORSE MUSSEL (3)
This is not a very handsome creature, for its shell is covered all over with a rather thick brown skin, which is very much wrinkled. It is quite common in many places, and yet one does not very often see it; for it is nearly always hidden underneath its byssus threads, which grow in thick masses. Besides this, it often burrows underneath the surface of the sand; so that unless you know just where to look for it, and how to look for it, you are not likely to find it.
1. INSIDE OF MUSSEL SHELL.2. THE MUSSEL.
3. THE HORSE MUSSEL.