EARTHWORM 95 



In such forms a very small portion will regenerate a complete worm. 



(W. f. 160). 



6. Nervous System 



The Earthworm possesses a much higher type of nervous system 

 than that of Hydra. It consists, as in the higher animals, of the 



CENTRAL NERVOUS SYSTEM and the PERIPHERAL NERVOUS SYSTEM. 



The former consists of a ventral nerve cord, which lies in the 

 coelom, ventral to the alimentary canal, in a median line. It 

 begins at the extreme anterior end of the animal and continues 

 throughout its entire length, enlarging in each segment to form a 

 ganglion. 1 In segment iv, just under the anterior end of the 

 pharynx, the cord divides to form a nerve collar, composed of 

 the ciRCUMPHARYNGEAL connectives, which completely encircles 

 the pharynx. On the dorsal side of the nerve collar, lying above the 

 pharynx in segment iii, is a bilobed swelling which constitutes 

 the cererral ganglion. This may be regarded as the rrain of 

 the animal. Running forward from the cerebral ganglion are two 

 pairs of nerves, from which a number of tiny branches arise to 

 innervate the prostomial region. (W. fs. 59, G; 66.) 



The peripheral nervous system consists of paired nerve trunks 

 which are given off from the nerve cord in each segment and which 

 innervate the various parts of the body. There is a definite 

 arrangement of these peripheral nerves in each segment which 

 we may now indicate, (a) Just back of the anterior septum in 

 each segment a pair of nerves is given off : one of the pair pene- 

 trates the body wall on the right side of the body, and the other 

 occupies a corresponding position on the left side of the body. 

 (6) From each of the segmental ganglia of the ventral nerve cord 

 two pairs of nerves arise, two of which run to the right side of the 

 body and two to the left, (c) The extreme anterior segmental 

 ganglion, located in segment iv just posterior to the division to 

 form the nerve collar, is known as the suresophageal ganglion, 

 and from this ganglion several pairs of nerves are given off which 

 aid in innervating the anterior segments. The two pairs of nerves 

 running anteriorly from the cerebral ganglion also belong to the 

 peripheral nervous system. 



Each of the peripheral nerves, except those arising in segments 

 iii and iv, run laterally for a short distance, then divide into a dorsal 



1 Plural, ganglia. 



