RADIATA. 191 



contains many instances totally unaccountable on any 

 scheme of material gradation whatever. The nettling 

 thread-cells, or Cnidce in the Hydroids, the peculiar alter- 

 nation of generations in the Medusae, the great amount 

 of muscular development in the Crinoida, the pedicel- 

 lariae, and the dental apparatus of Echinus, are all exam- 

 ples of structural arrangement for a purpose, and make 

 against the theory of evolutional development, or sur- 

 vival of the fittest. Each of these structures are the 

 most perfect of their kind, and seem to have no previous 

 structure from which they have developed, as they have 

 left no succeeding apparatus analogous or homologous 

 to them. 



