266 



METAZOAN PHYLA 



zoaea and the zoaea correspond also to no living forms, but the mysis 

 resembles very closely crustaceans belonging to the genus Mysis, which is 

 an ancient type still living. 



Fig. 164. — The developmental stages in the life history of a shrimp, Pcnaexis sp. 

 A, nauplius stage. B, protozoaea stage. C, zoaea stage. Z), mysis stage. Highly 

 magnified. {From Lang, " Text-book of Comparative Anatom,y," after Fritz Miiller.) 



Fig. 165. — An adult shrimp, Penaeus semisulcatus. X J^'. (From Huxley, "The Study of 



Zoology," after de Haan.) 



Because of this correspondence, which seems to show in the higher 

 forms a succession of stages recapitulating ancestral conditions, the 

 biogenetic law has also been termed the law of recapitulation. It has 



