MUSCLES OF THE EARTHWORM 145 



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 longi- 

 tudinal fibers and their contraction results in lengthening 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 points and formed from 

 special glands termed the seta-sacs. These are often of large 

 size and are conspicuous when the worm is opened. Between 

 the two setae of each pair a few longitudinal bundles of muscle 

 fibers help to strengthen the body wall and to complete the mus- 

 cular sheath. 



Other special muscles form the walls of the pharynx and by 

 their contraction and relaxation they shut and open this organ 

 thus making it a sucking pouch which draws in dirt, leaves, and 

 other extraneous matters. 



Still other special muscles form the walls of the gizzard making 

 it a grinding organ for cutting up food received from the crop. 

 Circular and longitudinal muscles, also form part of the wall of 

 the stomach intestine and by their successive contraction force 

 the enclosed undigested food materials toward the anus, thus 

 acting by peristalsis as do the blood-vessel muscles which form a 

 part of the walls. 



H. THE NERVOUS SYSTEM. The nervous system is closely 

 connected with the muscular system and it is well to get in mind 

 the muscle-nerve combination, for one always involves the 

 other, sensory cell, central nervous system and muscle all working 

 together in what is termed a " reflex action." 



The Sensory System. The cells of the skin are of different 

 kinds, the majority being epithelial or columnar cells inter- 

 spersed here and there with secreting or goblet cells which form 

 the slimy secretion poured out when the worm is irritated. In 

 addition to these there are many small sensory cells which 



