PHYLUM ARTHROPODA 



661 



special mode of locomotion, but the resemblances in many respects 

 are very strong. One of the most striking points of difference 

 is the indefiniteness in the number of the segments in the Myriapoda, 

 and their constant and definite arrangement in the Insecta. The 

 well-defined thorax of the Insects is wanting in the Myriapods in 

 general, but certain of the segments following the head differ from 

 the rest in various respects, and might be looked upon as con- 

 stituting a thoracic region. The presence in both groups of a 

 sharply marked-off head bearing antennae and jaws is an important 

 point of resemblance ; so is the absence in both of the voluminous 

 ' liver " of the Crustacea and Arachnida. The gap between the 

 two classes is narrowed by two converging groups the Symphyla 



Air- breathing 



Arac h nids 



Insecru 



MynoJDoda 



Onych 



Pycnogonida 

 Crusl-ocea 



Xifjliosura 

 Euryf^enda 



obora 



Pnmih've ArfhroJDods 



Anne lida 

 FIG. 571. 



among the Myriapoda on the one hand, and the wingless and in 

 other respects primitive Apterygota among the Insecta on the 

 other. 



While the Insecta thus appear to be nearly related to the Pro- 

 goneata, there are indications of relationship between the Opistho- 

 goneata and the Onychophora, and, through these, to the Chsetopoda. 

 The elongated, homonomously segmented body, the well-defined 

 head with its antennas, the occurrence of similar appendages on 

 all the body-segments, all point in this direction. Accordingly, 

 instead of placing the branchiate Arthropoda in one group and 

 all the air-breathing forms in another, and deriving the latter from 

 the former, we should probably express more correctly the affinities 



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