244 



GENERAL ZOOLOGY 



less specialization is evident than in either of the other 

 animals. The " brain" (supraesophageal ganglion, Fig. 128) 

 is a simple, very small, bilobed ganglion, joined by connec- 

 tives to a subesophageal ganglion ; from the latter a pair of 

 connectives extend along the ventral wall of the body cavity 

 to the posterior end, with a ganglion in every somite. The 

 brain and ventral nerve chain constitute the central nervous 



Cuticle of skin - - 

 Sensory nerve ganglion 



Sensory nerve fiber 



Anterior branch of 

 sensory nerve fiber 



===^ _'.'■"* Longitudinal muscle fibers 



Motor nerve fiber 



. - Posterior branch of sensory 

 nerve fiber 



Ventral nerve cord 

 Motor nerve ganglion 



Fig. 129. Arrangement of nerve fibers for reflex action in the earthworm 



Reconstructed from drawings by Havet 



system. In each somite three pairs of nerves run out to the 

 muscles of the body wall and the internal organs. The en- 

 tire set of nerves leaving the central nervous system con- 

 stitutes the peripheral (surface or outlying) nervous system. 

 If we were to trace one of the nerves outward from the 

 central nervous system as it penetrates the muscular tissue, 

 we should find that it divides into many very fine fibers. 

 Certain fibers can be traced to the end as their fine branches 

 become attached to a muscle fiber. The nerve fiber that is 

 united with a muscle fiber (Fig. 129) is a slender portion 



