48 



GENERAL ZOOLOGY 



groups. One remained aquatic, finally developing gills for 

 respiration in the water, and gave rise to the class Crustacea. 

 All the other arthropods became terrestrial and were hence 



^^^^ 

 Onycophora 



dncestral arthropod 



FIG. 28. A scheme to show how the arthropods have developed from their 

 ancient ancestor. The branches are not intended to represent actual relation- 

 ships, but to indicate the lines of specialization which have been followed. 



obliged to acquire the ability to breathe air. Most of the 

 air-breathing arthropods have developed a branching 

 system of minute tubes, the tracheae, which carry air to 



