2 THE FLEA [CH. 



The animals belonging to this large and important 

 collection, which compose the arthropod phylum, 

 have certain common characteristic features. We 

 find a body made up of a series of more or less 

 completely similar segments placed one behind the 

 other. In this they resemble certain worms which 

 are far less highly organised. The body is elongated, 

 symmetrical on either side, and the mouth and anus 

 are at opposite ends. There is, however, an import- 

 ant advance on the segmented worms. Each typical 

 segment carries a pair of appendages which are very 

 different from the foot-stumps that are found on 

 certain worms. These appendages of arthropods are 

 divisible into distinct limb-segments, separated from 

 one another by moveable joints, and acted upon by 

 special muscles. 



The common ancestor of all the various arthropods 

 which are found living on the earth to-day, was 

 probably composed of a series of segments each very 

 similar to the last and each bearing a pair of very 

 similar appendages. In the course of ages, these 

 appendages have been astoundingly modified in form 

 and in function. So it happens that we find in the 

 arthropods of the present day pairs of antennae, of 

 mandibles and other mouth-parts, of pincers, of legs, 

 of swimming-feet and of tail pieces which on close 

 examination can all be traced back to a common 

 structure. The body-segments, also, have been 



