BEING A WOIIM 



195 



going fibers which originate in nerve cells within the cord are known 

 as efferent or motor fibers, since they end in muscle cells and stimulate 

 them to contract, thus causing motion. The unit over which these 

 impulses travel is called a neuron, which is the term given to the nerve 

 cell and its prolongations. (See page 340.) In the earthworm sensory 



anterior- 



SerjSory c<=ll5{r<2cepto«) 

 epidermis.': 



■muscle cells 

 ;e|^fecton$) 



, ^—Septu.rrL \j 



•postsrior 



The nerve cord of the earthworm showing neurons concerned in the reflex 

 arc. Explain how adjustment to an unfavorable condition might be affected. 

 How might movement in another segment of the worm be co-ordinated with the 

 one shown in the diagram.'' (After Curtis and Guthrie.) 



impulses are passed longitudinally, both anteriorly and posteriorly, 

 by means of the peripheral nervous system, and these impulses are 

 modified by means of adjustor neurons in the central nervous system. 

 This accounts for the co-ordination between segments as the worm 

 crawls toward a desirable object or suddenly withdraws from a harm- 

 ful situation. 



The Reproductive System and Reproduction 



Earthworms have both testes and ovaries in the same animal, 

 and are therefore hermaphroditic, but they are not capable of self- 

 fertilization. Two pairs of testes lie attached to the anterior walls of 



