TRILOBITES AND OTHER FOSSIL CRUSTACEA. 177 



tions of the lower. In the Crustacea a very large 

 number of genera are alike in their youngest state. 

 From its resemblance to the adult condition of one 

 of the lowest of the crustaceans called Nauplius, this 

 state is usually called the " Nauplius stage." No other 

 group of animals passes through so many metamor- 

 phoses before reaching maturity, and each of these 

 is so well marked off from the rest, that it might be 



139. Asaphus caudatust 



Fig. 140. "Dudley locust' 



or Trilobite (Calymene 



Blutnenbachii). 



regarded as a generic type. Indeed, in many cases 

 genera have been founded on these distinctions, so 

 that the same animal, at different periods of its life 

 has been regarded not only as a distinct species, but 

 as belonging to another genus. The young of the 

 common lobster, for instance, passes through at least 

 six stages, which are so unlike each other that only 

 careful observation has settled they are not different 



N 



