CONCLUSION. 273 



In accordance with these principles, the linear distribution of functions in the 

 following order: sensory, feeding, respiratory, and reproductive, becomes very 

 early established, and thereafter suffers but minor modifications throughout the 

 entire range of the arthropod-vertebrate series. For example, in the arachnids the 

 procephalic appendages are mainly sensory, the dicephalic, masticatory, the meso- 

 cephalic, locomotor, and the metacephalic, respiratory and reproductive. The 

 boundaries to these divisions are not always sharply defined, and there may be 

 some overlapping of functions, but not enough to invalidate the general law. 



One of the more recent changes in this primitive distribution of functions to 

 the appendages was the transfer of the locomotor organs to a postcephalic position, 

 owing largely to the increased number of metameres and the consequent shifting 

 of the center of balance backward. 



18 



