There are several different kinds of Razors, and one of them is called the “sabre razor,” because its shells are curved, just like the scabbard of a sabre. It is fairly common, but you are never likely to find its burrows, unless you go to look for them just at low-water after a spring-tide, because it almost always lives below the ordinary low-water mark. But after spring-tides--which come twice in every month, once when the moon is new and once when it is full--the waves retreat much farther than they do at other times. Then, if you go right down to the water’s edge, you may often find creatures which you will never meet with higher up on the beach. And one of these is the sabre razor. PLATE XX THE PINNA This is the largest of all the shell-bearing molluscs which live in our British seas, for it has been known to reach a length of nearly two feet. It is found chiefly on our southern coasts, and always lies upright, half buried in the mud at the bottom of the water, with its shells partly opened. And it always fastens itself down by a bunch of “byssus” threads, like those of the mussel, which are so strong that it takes a very hard pull indeed to tear them away from their hold. In the British Museum you may see a pair of gloves which have been made out of the byssus threads of a pinna, and if these creatures were more plentiful their threads would no doubt be used in this way very largely indeed. Now why do you think that the pinna always rests at the bottom of the water with its shells partly opened? [Illustration: PLATE XX THE PINNA.] Well, the reason is a very odd one. It is setting a trap for fishes! For fishes, as perhaps you know, are very inquisitive creatures. They always want to know all about everything, and whenever they see a hole they think that they must find out what is inside it. So when a little fish comes swimming past a pinna, and catches sight of its gaping shells, it is almost sure to venture in between them. Then the shells close tightly, and it finds itself in a prison from which there is no escape; and very soon it is killed and devoured. In colour, the shells of the pinna are very pale brown, and a number of ridges run down it from the smaller end to the larger. When the animal is full-grown it is sometimes not at all easy to see its shells, for they are covered almost all over with barnacles and the tubes of sea-worms. CHAPTER IV CRABS HOW CRABS GROW If you hunt about in the pools among the rocks when the tide goes out, and look behind the masses of sea-weeds which cover them, you are quite sure to find a good many crabs of several different kinds. Before I tell you about these, however, I think you would like to know something about the way in which these curious creatures grow.