CHAPTER IX 



ARACHNIDA INTRODUCTION 



THE Arachnida, together with the Crustacea, Insecta, Myriapoda, 

 and Peripatus, make up the great phylum Arthropoda, a phylum 

 which, from the point of view of numbers of species and of 

 individuals, is the dominant one on this planet, and from the 

 point of view of intelligence and power of co-operating in 

 the formation of social communities is surpassed but by the 

 Vertebrata. The Arachnida form a more diverse class than 

 the Insecta ; they differ, perhaps, as much inter se as do the 

 Crustacea, and in structure as in size and habit they cover a 

 wide range. 



Lankester in his article upon the Arthropoda, in the tenth 

 edition of the Encyclopaedia Britannica, dwells upon the fact 

 that whereas the adult Peripatus has but one persisting seg- 

 ment in front of the head, and its mouth is between the 

 second persisting appendages, in Arachnids the mouth has receded 

 and lies between the bases of the appendages (pedipalpi) of the 

 third persisting segment, while two of the persisting segments, 

 those of the eyes and chelicerae, have passed in front of the 

 mouth. This process has continued in the Crustacea and in the 

 Insecta ; in both of these classes there are three embryonic 

 segments in front of the adult mouth, which lies between the 

 appendages of the fourth segment. 



In the larger and more complex Arachnida the number of 

 segments is fixed and constant, and though possibly no adult 

 member of the group, owing to the suppression of one or more 

 segments during the ontogeny, ever shows the full number at any 

 one time, the body can be analysed into twenty- one segments. It 

 is interesting to note that the same number of segments occurs in 



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