TRILOBITES AND OTHER FOSSIL CRUSTACEA. 205 



It has frequently been pointed out that by care- 

 fully studying the embryological development of any 

 individual up to its adult state, we may get a glimpse 

 of the stages through which the genus or species, or 

 even the family, has passed in its evolutional develop- 



Fig. 164.— Phases in the Larval Development of the Crab : a, Zcea. stage ; />, advanced 

 (or megalopa) stage ; c, earliest completed stage. 



ment. Thus the young larvse of the common crab — 



whose aborted and contracted body is popularly 



known as the " apron " — in the first three or four of 



its early stages, has a properly jointed 



body like the lobster. The long-bodied 



crustaceans preceded the short-bodied 



ones in order of geological time ; so 



that we have here an indication that 



the short-bodied crabs are descended 



from long-bodied and lobster-like 



ancestors. 



The short-bodied crustaceans (called Brachyura on 



Fig. 165. — Carapace of 

 Notopocorystes. (Cam- 

 bridge Greensand). 



