NERVOUS SYSTEM OF THE EARTHWORM 149 



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, while smaller muscles connect the adjacent setae 

 on each side (Fig. 59 s.m.). 



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, mak- 

 ing 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 contrac- 

 tion 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, with 

 secreting or goblet cells interspersed here and there, which form 

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

 addition to these, there are many small sensory cells which 

 receive stimuli when the worm is irritated. These are much 

 more numerous in the anterior part of the worm than elsewhere, 

 making this the most sensitive or irritable portion of the whole 

 body. The irritation received by these sensory cells is passed 

 as a nervous impulse through prolongations of the sensory cells 

 in the form of nerve fibers to the central nervous system which 

 runs from end to end of the worm. There are no aggregates of 

 sensory cells to form sense organs, but the entire skin is sensitive. 



The earthworm marks a great advance in the organization of 

 the nervous system. In Hydra and the coelenterates there is 



