CHAPTER: WX 
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 hes 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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