148 



ORGANS AND ORGAN SYSTEMS 



inner wall consists of longitudinal fibers running from somite to 

 somite, and their contraction results in drawing head and 

 tail end together, or in shortening the worm (Fig. 59). The 

 muscle fibers are closely packed together, giving the appear- 

 ance of many muscles. The outer wall consists of fibers 

 running around the somite at right angles to the longitudinal 



FIG. 59. Transverse section of the earthworm behind the clitellum. a.c., 

 Cavity of the digestive tract; c, cuticle; coe, coelom; c.m. t circular muscles; c.v tj 

 parietal vessel; d.v., dorsal vessel; hy, hypodermis; l.m., longitudinal muscles; 

 n.c., ventral nerve chain; p.e., peritoneal endothelium; s, seta; s.g., setigerous 

 gland; s.i.v., ventral vessel; s.m., muscle connecting two groups of setae on the 

 same side; ty, typhlosole. (From Sedgwick and Wilson.) 



fibers, and their contraction results in shortening the diameter 

 of the somite and thus in elongating the worm. Both sheaths 

 of muscle are broken in four places at points where setae are 

 formed, and here special muscles for moving the inner ends 

 of the setae are developed; by their contraction the setae are 

 moved one way or another. These setae are lifeless rods of 

 chitin, somewhat sharpened at the outer ends and formed from 



