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. [Illustration: PLATE XIV 1. INSIDE OF MUSSEL SHELL. 2. THE MUSSEL. 3. THE HORSE MUSSEL.] But if you go down to the pools at the very edge of the water when the tide is quite low, and scrape away the sand which is heaped up against the bottom of the rocks, you may very likely come upon quite a large cluster of these curious creatures. Horse mussels are not used for food as common mussels are, because they have a very strong and unpleasant taste. PLATE XV THE VARIABLE SCALLOP (1) A good many different kinds of scallops are found on our shores. One of them--the Common Scallop--is as large as the palm of a man’s hand, and is used for food. You may often see it in fishmongers’ shops. But you are not at all likely to find its empty shells lying on the shore, for it lives in rather deep water. You may find those of the Variable Scallop, however, very often indeed in places where the shore is sandy. It is called the “variable” scallop because it varies so much in colour that one hardly ever sees two of its shells which are quite alike. Sometimes they are crimson, sometimes pink, sometimes mauve, sometimes dark yellow, sometimes golden yellow, and sometimes blotched and mottled with different colours. A number of ridges run down the shell from the hinge to the margin, and on each of these is a row of short spikes; so that the animal looks something like a tipsy-cake!