106 DIVERSIONS OF A NATURALIST 



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 antennae — 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, power- 

 ful 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 move- 



