The cockle has a "foot" of a very curious scythe-like shape, usually carried bent up between the two valves of the shell. Those who rightly like to confirm statements about unfamiliar animals can do so by buying a cockle or two at the fishmonger's. Some bivalves (the Noah's-ark-shell, called "Arca," and a few others) have a great flat foot, like that of the univalves, and crawl about on it. But in most bivalves it is curiously elongated and modified, for the purpose of burrowing into sand by vigorous strokes, and in some it is suppressed altogether, as in the oyster. The cockle is remarkable for the fact that when placed on a board or a rock it will give such a vigorous kick with its bent foot as to throw itself up a yard or so into the air. A naturalist (Stutchbury) dredging in Port Jackson, Australia, many years ago was overjoyed at discovering in his net three specimens of a very peculiar kind of cockle (Trigonia), which was till then only known in the fossil state from the oolite strata of Europe. He placed the three novelties on the seat of his boat, and was looking at other things when he heard a click-like sound, then another. He turned his head and saw that two of his newly-discovered "living fossils" had jumped overboard, and had the pleasure of seeing the third perform the same feat! CHAPTER XVII SAND-HOPPERS WHEREVER there is a sandy seashore with here and there masses of dead seaweed and corallines thrown up by the waves, you will find sand-hoppers feeding on the debris. They are crustaceans, like crabs, shrimps, and barnacles, but in general aspect resemble enormous fleas. I hope that this comparison will not enable any reader at once to picture the less familiar by the more familiar. A good-sized sand-hopper is about half an inch long, and jumps not by means of a specially large pair of legs as the flea does, but by the stroke of the hind body, the jointed rings of which are carried curled downwards and ready to give a sudden blow. The sand-hopper (Fig. 20, a) has some of the rings or segments of the mid-body distinct, and not fused with those of the head or overhung by a great shield as in the lobster, crab, and shrimp. His walking legs and jaw-legs are also not quite of the same shape, though similar to those of a lobster, and his two little black eyes are not mounted on stalks, but are flush with the surface of the head. There are two quite distinct kinds of sand-hopper which live in crowds together on our sandy shores. They are not very different, but still are distinguished by naturalists from one another; one is called Talitrus (Fig. 20, a), the other Orchestia (Fig. 20, b). They are very similar in appearance and structure to a fresh-water creature common in weedy streams, which has no English name (except the general one of "fresh-water shrimp"), and is called by naturalists Gammarus.