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.
It has been discovered that in the growth from the egg of many crustaceans the young hatches out as a "nauplius" with only three pairs of legs. The front two pairs later gradually grow to be the feelers, the third pair become eventually the mandibles or first pair of jaw-legs. These legs all present themselves at first as active, powerful swimming "oars," beset with peculiar feathery hairs and not in the shape which they later acquire. The kite-shaped nauplius baby-phase, smaller than a small flea, with its three pairs of violently jerking legs, is a very important little beast. It is the existence of this young stage in the growth of barnacles and sea-acorns which has demonstrated that they are crustaceans, that is to say, belong to the crab class. The fixed shell-like barnacles and sea-acorns hatch from their eggs each as a perfect little "nauplius," like that drawn in Fig. 9. They swim about with jerking movements caused by the strokes of the two front legs and of the pair which will become the mandibles. Their limbs have the special form and are beset with the feather-like hairs, and the whole creature has the kite-like shape--characteristic of the nauplius young of other Crustacea. They are indeed indistinguishable from those young. Whilst it was the Army doctor, Vaughan Thompson, who discovered that barnacles are strangely altered "shrimps," it was Darwin who made one of the most interesting discoveries about them--a discovery of which he was always, and rightly, very proud--as I will explain in the next chapter.