PARALLELISM. 203 



invertebrate forms, and the nauplius and zoaea of the 

 Crustacea. 



Such are the statoblasts which are resting-stages 

 for the embryos of fresh-water sponges and Polyzoa, 

 and the glochidia of the Unionidse, which are wanting 

 in the marine forms of the same orders. Such are the 

 amnion and allantois of certain Vertebrata and the 

 placenta of certain Mammalia, which have no refer- 

 ence to any structures but their own residua, found in 

 the adults of those animals. 



A remarkable instance of this state of things ap- 

 pears in the history of the evolution of the insects. It 

 is quite impossible to understand this history without 

 believing that the larval and pupal states of the high- 

 est insects are the results of a process of degeneracy 

 which has affected the middle periods of growth, but 

 not the mature results. The earliest insects are the 

 Orthoptera, which have active aggressive larvse and 

 pupae, undergoing the least changes in their meta- 

 morphosis (Ametabola), and never getting beyond the 

 primitive mandibulate condition at the end. The meta- 

 morphosis of the jsiwed Neuroptera is little more 

 marked, and they are one of the oldest orders. 



The highest orders with jaws undergo a marked 

 metamorphosis (Coleoptera, Hymenoptera), the Hy- 

 menoptera even requiring artificial intervention in 

 some instances to make it successful. Finally, the 

 most specialized orders, the suctorial Diptera and 

 Lepidoptera, especially the latter, present us with very 

 unprotected more or less parasitic larval stages, both 

 active and inactive. These animals have evidently de- 

 generated, but not so as to prevent their completing a 

 metamorphosis necessary for purposes of reproduction. 

 As is well known, many imagines (Saturniidse, CEstridas) 



