150 THE APODID/E PARTI 



tion. In this book, however, we have arrived at 

 such a conclusion from quite another point of 

 view. We started by endeavouring to show that 

 Apus, from its many striking Annelidan charac- 

 teristics, was a transition form between the Crus- 

 tacea and the Annelida, and hence a primitive 

 Crustacean. It comes, therefore, as no slight sup- 

 port to our argument to be able to show that the 

 higher Crustaceans pass through an Apus-like stage. 

 That all Crustacea do not pass through this stage 

 is easily explained by the theory of abbreviated 

 development, so that this stage is either passed 

 through in the egg, or else considerably disguised 

 by the early acquirement of adult characteristics. 

 As to the case in which the stage is passed 

 through in the egg (e.g. among the Malacostraca) 

 it is important to note that this is not the case 

 in all Malacostraca, a Nauplius stage occurring, 

 for example, in the development of Penaeus and of 

 Euphausia. 



The theory of the origin of Apus from an Annelid 

 gives us at once the true relation of the Nauplius to 

 the Trochophora. It is not necessary to assume that 

 Apus passes through a Trochophora stage, because 

 this latter is a stage in the development of the 

 Annelid specially adapted to a free-swimming larval 

 life. The equivalent stage in Apus, being no longer 

 larval but embryonic, does not require to develop 

 the special characteristics of the Trochophora. 



When the young Crustacean is hatched as a 

 Nauplius, it has already advanced considerably 



