CHAPTEK XII 

 AKTHKOPODA 



External Characteristics. — The Arthropoda include a 

 great number of apparently divergent forms such as lob- 

 sters, spiders, flies, but their common characteristics are 

 well-defined. The name means jointed-foot, and the 

 class includes those animals which have an external 

 jointed skeleton and jointed appendages. As in the 

 segmented worms an evident' external segmentation is 

 associated with an internal segmentation, that is, the 

 body is divided into a series of segments each of which 

 contains parts of the internal organs. Many of the seg- 

 ments are modified, however. Some of them are over- 

 developed, some are under-developed and some are 

 fused. The result is that different regions in the body 

 become marked off, and it is not always easy to deter- 

 mine the number of segments that are involved. Usu- 

 ally three regions may be distinguished, the head which 

 has well-developed sense organs, the thorax which bears 

 the organs of locomotion, and the abdomen which may, 

 or may not, bear appendages and may, or may not, 

 show marked segmentation. 



Nervous System. — These animals are highly devel- 

 oped and their tissues well differentiated. They are very 

 irritable and respond readily to outside stimuli through 



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