PHYLUM ARTHROPODA 



303 



FIG. 225. Larva of lobster in Mysis stage. 

 (From Sedgwick, after Sars.) 



a two-layered sac 

 something like a 

 gastrula, which he 

 called a Gastrcea. 

 The ccelenterates 

 w T ere considered to 

 be gastraea slightly 

 modified. 



Fritz M tiller de- 

 rived strong arguments in favor of biogenesis from a study of 

 certain CRUSTACEA belonging to the MALACOSTRACA. Many 

 members of this group do 

 not emerge from the egg 

 so nearly like the adult as 

 does the crayfish. The 

 lobster, for example, upon 

 hatching (Fig. 225) re- 

 sembles a less specialized 



FIG. 226. Two stages in the development of the shrimp, Penaus. 

 A, Nauplius stage. B, Protozoaa stage. (From Sedgwick's Zoology, after 

 Fritz Muller.) 



