CHAPTER 26 



THE PHYLUM ARTHROPODA 



No phylum of living animals shows such diversity of form and 

 number of individuals as does the Arthropoda. Close to a million 

 different species are at present recognized, and many more are un- 

 doubtedly remaining to be discovered. 



Many of the characteristics which contribute to the success of the 

 arthropods as a group were first seen among the annelids : the seg- 

 mented body, the paired appendages, and the anterior sense organs. 

 Among the arthropods, these basic characters have been developed to 

 a higher degree, and, in addition, new features have been added. These 

 new features are the development of a heavily chitinized exoskeleton 

 which protects the animal and gives surfaces for muscle attachment and 

 the jointed appendages which allow great freedom of movement. The 

 inflexible outer armor is arranged in plates corresponding to the seg- 

 ments, and thus flexibility is achieved with maximum protection. The 

 modern arthropod can best be compared to the medieval knight encased 

 in his heavy hinged armor. 



In their evolution, the arthropods have radiated into types that 

 are able to live in all conceivable habitats. Except for the snails, the 

 arthropods are the only invertebrates that have succeeded as completely 

 terrestrial animals. Their exoskeletons, respiratory devices, and habits 

 are all important in this conquest. Many of the arthropods are so 

 specialized in their habitat requirements that they may be found only 

 in very limited areas, while others are highly adaptable forms. 



The members of this group can be counted among the most noto- 

 rious and familiar of the invertebrate phyla. Some may be considered 

 as man's greatest enemies inasmuch as they compete with him for both 

 food and shelter; others may be considered the most useful of his allies. 

 The insects, spiders, crayfish, crabs, mites, millipedes, centipedes, and 

 a host of less familiar forms are all arthropods. 



486 



