THE EVOLUTION THEORY 



133 



In the evolution of all lower crustaceans, the so-called Ento- 

 mostraca, we meet invariably (however differentiated the adult 

 animals may be) with' a. peculiar larva, the Nauplius. It is a 

 lively little animal which swims gaily about by means of its three 

 pairs of appendages, of which the first is unforked, the other 

 pairs doubly-branched. The median eye is also a distinctive 

 feature in the Nauplius larva. By 

 a more or less complicated meta- 

 morphosis the Nauplius changes 

 into an extremely great variety of 

 crustacean forms. And as ' duck- 

 mussels' also originate from Nauplius 

 larvae their position in the system 

 becomes clearly defined. 



It is here necessary to modify to 

 some extent the preceding statement 

 that all crustaceans pass through a 

 Nauplius stage, for an exception is 

 made by the water-flea (Daphnia), 

 the favourite food of aquarium 

 fishes. This little crustacean passes 



the stage of the free-swimming Nauplius in the ovum and 

 emerges as an adult Daphnia. 



It is a well-known fact that the growing larvae and young 

 crustaceans moult several times. The object of the moulting is 

 chiefly to enable the animal to grow, for the rigid chitinous 

 cuticle w r ould make development impossible. Before the moulting 

 the old shell becomes virtually dead, a new shell begins to form 

 within the old, and finally the shell is cast with considerable 

 effort. Almost at the moment of casting the anirnal, hitherto in 

 a compressed state, increases in size. The new shell is at first 

 soft and requires several hours to acquire firmness. When it 

 has become rigid the growth of the crustacean is once more 

 postponed till the next moult. 



However strange it may sound, the Nauplius in the ovum 

 performs a similar act of moulting, a process which is all the 

 more illusory as the rigid egg-shell effectually precludes every 

 increase in size. Is not the only explanation this, that the 

 Daphnia, like other crustaceans, had once a free-swimming 



PIG. 43. NAUPLIUS STAGE OF 



Pen&us potimirum. 



