i66 



AN INTRODUCTION TO ZOOLOGY 



85, set. New setae may be produced by any one of a number of 

 cells lying at the bottom of the epidermal sacs. On mature 

 worms the setae on somite XXVI are enlarged and modified as 

 sexual setae. 



dors.V 



Tiep 



nephrost 



n.co 



set 



vent.r 



FIG. 85. Transverse section through the middle region of the body of Lum- 

 bricus. circ. mus., circular muscle fibers ; ccel., ccelom ; dors, v., dorsal 

 vessel; epid., epidermis ; ext. neph., nephridiopore ; hep., chloragogen 

 cells; long, mus., longitudinal muscles; neph., nephridium ; nephrost., 

 nephrostome ; n. co., nerve cord; set., setae; sub. n. vess., sub-neural 

 vessel ; typh., typhlosole ; vent, v., ventral vessel. (From Parker and 

 Haswell after Marshall and Hurst.) 



The outer covering of the body is a thin transparent membrane, 

 the cuticle (Fig. 85, cut.), which is secreted by the cells lying just 

 beneath it. The cuticle protects the body wall from physical 



