448 THE WONDER OF LIFE 



and wholly reproductive, using the energy accumulated 

 in the larval period ; but in the Mermithidae the state of 

 affairs is even more striking, for the energy accumulated 

 in the second larval stage serves not only for mature life 

 and for reproduction, but also for the first chapter in the 

 life of the next generation ! The black horse-hair worms, 

 hundreds of which are sometimes seen in a little wayside 

 pool, each about the thickness of a hair from a horse's tail, 

 have a somewhat similar life-history. The minute larvae 

 enter water-beetles and other insects and grow large within 

 them, to a length of four inches or so, much longer indeed 

 than their hosts. When they become mature they work 

 their way out of the insects and sometimes suddenly appear 

 in large numbers in the pools. We have seen a pool a 

 couple of feet across, so crowded with them that over a 

 hundred could be lifted in a handful, just like a bunch of 

 vitalized hairs, as the mediaeval naturalists believed them 

 to be. 



Barnacles and Acorn -Shells. The barnacles (Lepas, 

 etc.) on floating timber and the acorn-shells (Balanus) 

 encrusting the shore rocks are much alike in their life- 

 history, and a very remarkable one it is. Out of the egg 

 of the barnacle there emerges a minute free-swimming 

 larva a Nauplius with three pairs of appendages, an 

 unpaired eye and a delicate dorsal shield. After moulting 

 several times, it fixes itself by means of its first pair of 

 feelers, which have become suctorial, to some floating 

 object, and secures its adhesion by a secretion of gluey 

 material. The anterior end by which it has fixed itself 

 is drawn out into a long flexible stalk, and a thorough- 

 going change occurs in the bodily structure, until the final 

 form is reached. During this metamorphosis the animal 



