162 



THE EVOLUTION THEORY 



«wimming-legs, to different kinds of functions, one becoming an 

 antenna, another a jaw or a swimming-arm, a third, fourth, fifth, and 

 so on, a jumping-leg, a copulatory organ, an egg-bearer, a gill-bearer, 

 or a tail-fin. 



That the development has in general followed those lines is made 

 clear chiefly by the fact that the members of all these different orders 

 of Crustaceans still arise from nauplius larvae, even in those cases in 

 which the perfect animal possesses a structure differing widely from 

 the usual Crustacean form. All Crustaceans arise from the nauplius 



Fig. 109, Metamorphosis of one of the higher Crustacea, a Shrimp 

 (Peneics potimirim), after Fritz Miiller. A, the nauplius larva with the three 

 pairs of appendages : I, the antennae ; II and III, the biramose swimming- 

 feet. Au, the single eye. B, first Zosea stage, with six pairs of appendages 

 {I-VI). Skn, area where new segments are being formed. 



form, even those of the higher orders, though they may not arise from 

 a nauplius larva. But this very circumstance, that in most of the 

 higher and many of the lower Crustaceans, the young animal, when 

 it emerges from the egg, already possesses more numerous segments 

 and limbs than a nauplius larva, again points to the connexion 

 between phylogeny and ontogeny, for in these cases the nauplius 

 stage is gone through ivitJdn the ovum. The whole difference between 

 this and the forms we considered first lies in the fact that, in the 

 latter, the development is greatly shortened, condensed, as we might 



