318 



THE STUDY OF ANIMAL LIFE CHAP. 



a Zoaea, grows and moults and becomes a Mysis, grows 

 and moults and becomes a Penceus (see figs. 105 to 108). 

 Now, these life-histories are hardly intelligible at all 

 unless we believe that Penceus does in some measure 

 recapitulate the steps of racial progress, that the crab 

 does so to a slighter extent, that the lobster has abbrevi- 

 ated 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 



FIG. 105. LIFE-HISTORY OF Penseus ; THE NAUPLIUS. 



Crustacea perhaps not much less simple than the Nau- 

 plius larvae which many of them exhibit. In the course 

 of time some pushed forward in evolution and attained 

 to the level of structure represented by the Zcsea larva?. 

 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 



