PHYLUM A&TH&OPODA 



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- breaking 

 Arach nids 



Insecta 



Mynofjoda 



Onycho 



Pycnogonida 

 Crustacea 



Xi[jliosura 

 Euryfjfenda 



Primih've Arrhrofsods 



Annelida 



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 antennae, 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 



VOL. i u u 



