490 



ZOOLOGY 



SECT. 



of the fact that, while comparatively thick and strong in places where 



no movement is required, 

 it is thin and flexible in 

 the intervening spaces, and 

 thus allows of the move- 

 ment of the harder parts 

 upon one another. 



The seta?, which occur 

 on many parts of the body, 

 and in particular fringe 

 the appendages, are hollow 

 offshoots of the chitinous 

 cuticle, containing a proto- 

 plasmic core continuous 

 with the epidermis (Fig. 

 399). They thus differ 

 fundamentally from the 

 setae of Cha-topods, which 

 are solid rods sunk in 

 muscular sacs. 



The muscular system 

 is well developed (Fig. 

 390). Underlying the epi- 

 dermis is a layer of con- 

 nective tissue, and beneath 

 this is found, in the pos- 

 terior or limbless part of 

 the abdomen, a layer of 

 longitudinal muscles (Fig. 

 390) encircling the body, 

 and attached by connec- 

 tive-tissue to each seg- 

 ment. In this way the 

 muscular layer is itself 

 segmented, being divided 

 by the connective-tissue 

 insertions into muscle-seg- 

 ments or myomeres. The 

 action of these muscles is 

 to approximate adjacent 

 segments : according as the 

 fibres on the dorsal, ven- 

 tral, or lateral regions con- 

 tract, the abdomen will be 

 raised, lowered, or turned 

 sideways. In the limb- 

 bearing portion of the abdomen and in the thorax there is no 



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