174 



SEGMENTED WORMS— THE ANNELIDS 



through your fingers you can easily tell the direction in which these little 

 bristles point by the traction on your fingers. 



The nervous system of the earthworm is organized very much like 

 that of the higher animals, including man. There is a brain at the ante- 

 rior end of the body and a nerve cord running the length of the body 

 giving out nerve branches at different levels that run to all parts of the 

 body. Some of these are sensory nerves and bring impulses only from 

 sense organs to the nerve cord and continue up the cord to the brain. 

 Others are motor nerves and lead from the brain and nerve cord out to 

 the muscles and stimulate them to react. However, in the earthworm, 

 the nerve cord is on the ventral rather than the dorsal body wall and the 

 reactions are more or less of the reflex type with the brain having com- 



Brain 



Circumpharyngeal Subpharyngeal Nerve 

 connectives ganglion 



Nerve Ganglion 

 cord 



Fig. 12.3. Diagram of the central nervous system of the earthworm as seen from the 



side and slightly above with the left body wall removed. The anterior part of the 



digestive system is shown in outline to indicate relative positions of the organs. 



paratively little to do with correlating the body reactions. A cross- 

 section of the nerve cord will reveal three giant fibers in the dorsal re- 

 gion of the cord. These fibers contain nerves running the length of the 

 body and connecting the different segments, so that they work in a co- 

 ordinated manner, and strong stimulation brings response from all seg- 

 ments even though the brain has been removed. This is possible be- 

 cause each segment contains an enlargement of the nerve cord called a 

 ganglion which acts somewhat like a little brain for that segment and 

 makes the proper connections for response to stimuli. An earthworm 

 can be cut into several parts and each part will go right on crawling 

 around for a time and reacting to stimuli much as if nothing had hap- 

 pened, for the nerves in the giant fibers connect the different ganglia of 

 each part together and give it coordination. 



In higher forms of animal life the brain assumes a much greater con- 

 trol over the body and the reflexes from the spinal cord level are much 



