AETHEOPODA. 405 



CHAPTER X. 



ARTHROrODA. 



Laterally symmetrical animals with heteronomously segmented lody 

 and jointed seymental appendages ; with brain (suprao&sopliageal 

 ganglia) and ventral nerve cord (ganylionic chain]. 



The most important characteristic which distinguishes the Arthro- 

 poda from the closely allied segmented worms, and is an essential 

 condition of a higher organization and grade of life, is the possession 

 of jointed segmental appendages which serve as organs of locomotion. 

 In place of the unjointed parapodia of the Chcetopoda, jointed 

 appendages more adapted for locomotion and confined to the ventral 

 surface, are present. Every segment may possess a ventral pair of 

 appendages which, in the simplest case, are short and consist of only 

 a few joints (Peripatus) (fig. 325). While in the Annelida loco- 



FIG. 325. Ptripafut ciijiciiKia (after Moseley). 



motion is effected by the movements of the segments and undulatory 

 movements of the whole body, in the Arlhropoda the function of 

 locomotion is removed from the chief axis of the body to the 

 secondary axes, i.e., to the paired appendages, with the result of the 

 possibility of a much more effic'eat discharge of the function. The 

 appendages enable the Artliropoda not only to swim and creep with 

 much greater ease and speed, but also to execute various kinds of 

 more complicated movement, e.g., running, climbing, springing, and 

 flying. The Arthropoda are, therefore, true terrestrial and aerial 

 animals. 



The high development of the organs of locomotion as paired 

 appendages leads of necessity to a second essential property, viz., to 

 the heteronomy of the segmentation, and in connection with this to the 

 hardening of the outer layer of the skin to form a firm exo-skeleton. 

 If the function of the limbs is to be perfectly discharged, there will 

 be need of a considerable mass of muscle, the points of attachment 

 of which can only be furnished by the integument of the body. 

 The insertions of the appendages and their muscles, therefore, require- 



