56 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I31 



Gurney (1942) has pointed out that "the larval stages of today pro- 

 vide evidence for phylogeny, but indirectly," since the ontogeny of 

 an animal recapitulates the ontogeny of its ancestors. 



Fig. 21. — Decapoda: Penaeidea. Developmental stages of Pcuacus sctifents (L."). 

 (A-F from Pearson, 1939.) 



A, first nauplius. B, metanauplius. C, first protozoea. D, tliird protozoea. E, 

 first mysis. F, first postmysis. G, adult. 



2Mxpd, second maxillipeds ; Prpds, rudiments of pereiopods. 



It is true that ontogenetic stages of a species may represent in a 

 modified way adult ancestral stages of phylogenetic evolution. The 

 odult ancestry of a crustacean, however, can go back only as far as 



