532 



ZOOLOGY 



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 setae, 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. 

 433). They thus differ 

 fundamentally from the 

 setoe of Chsetopods, which 

 are solid rods sunk in 

 muscular sacs. 



The muscular system 

 is well developed (Fig. 

 424). 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 encir- 

 cling the body and attached 

 by connective-tissue to each 

 segment. In this way the 

 muscular layer is itself seg- 

 mented, being divided by 

 the connective-tissue inser- 

 tions into muscle-segments 

 or myomeres. The action 

 of these muscles is to ap- 

 proximate adjacent seg- 

 ments : according as the 

 fibres on the dorsal, ventral, 

 or Hteral regions contract, 

 the abdomen will be raised, 

 lowered, or turned side- 

 ways. In the limb-bearing 

 portion of the abdomen and 

 in the thorax there is no longer a continuous muscular tube, but 



