CHAP. XII The Life-History of Animals 



199 



Now these life -histories are hardly intelligible at all 

 unless we believe that Penceus does in some measure recapi- 

 tulate the steps of racial progress, that the crab does so 

 to a slighter extent, that the lobster has abbreviated its 

 obvious recapitulation much more, while the crayfish has 

 found out a short cut in development. Let us exercise our 

 imagination and think of the ancestral Crustacea perhaps 

 not much less simple than the Nauplius larvae which many 



Fig. 39. — Life-history of Pentsus ; the Nauplius. 



of them exhibit. In the course of time some pushed for- 

 ward in evolution and attained to the level of structure 

 represented by the Zoea larvas. At this station some 

 remained and we have already mentioned the " water-flea " 

 Cyclops as a crustacean which persists near this level. But 

 others pushed on and reached a stage represented by 

 Mysis, and finally the highest crustaceans were evolved. 



Now to a certain extent these highest crustaceans have 

 to travel in their individual development along the rails 

 laid down in the progress of the race. Thus Penceus, 



