Following on the jaw-legs or foot-jaws and walking-legs, in the three crustaceans we are looking at, comes the jointed tail or hind-body, consisting of seven pieces. The first five rings of the tail have small =Y=-shaped legs, a pair to each ring. They are called "swimmerets," whilst the sixth has legs of the same shape, but very large and flat. In the middle between these large flat legs is the last ring, which has no legs, but is perforated by the opening of the intestine. You will see if you compare the crab and the lobster (or the prawn, which is very much like the lobster), that the crab has the so-called head (really head and mid-body combined) drawn out from side to side, so as to make it much wider than it is long. And, moreover, the jointed tail or hind-body seems at first sight to be absent in the crab. But if you turn the crab (a dead one) on his back, you will find that he has a complete tail, on the whole like that of the lobster, but pointed and bent forwards, and closely packed under the fused head and mid-body in a groove, from which you can raise it and turn it back. [Illustration: FIG. 9.--The larval or young form of Crustacea known as "the Nauplius." This is the "Nauplius" of a kind of Prawn. The three pairs of branched limbs are well seen. Much magnified.] We have not yet done with the various forms assumed by the legs of our three crustaceans--for, actually in front of the mouth, there are two pairs of peculiarly altered legs. Originally in crab-ancestors, and at the present day in the very minute young stage of growth called "the Nauplius" (Fig. 9), the mouth was not behind these two front pairs. It has sunk back as it were, gradually moved so as to leave the legs in front of it. As we now see them in the crab, lobster, and prawn, the two pairs of legs in front of the mouth are jointed filamentous things--the feelers or antennæ--very long in prawns and lobsters, short in crabs. In the ancestors of crabs, lobsters, and prawns these feelers were undoubtedly swimming legs. In the "nauplius" stage (Fig. 9) of some prawns, and in many minute crustaceans often called "water-fleas," we find these feelers not acting as mere sensory organs of touch, but relatively strong and large, with powerful muscles, striking the water and making the little creatures bound or jump through it in jerks.