NERVOUS SYSTEM AND SENSE-ORGANS 



H.L 



Brain- 



of the body (fig. 1009). In the lower Annelids this system 

 is closely connected with the ectoderm or outer layer of the 

 skin, but in the more specialized members of the group it has 

 sunk within the muscular layers of the body-wall, where it is 

 much better protected. Connected with these central organs 

 are a large number of slender nerves, that come into intimate 

 relation with the various organs of the body, and are made up 

 of excessively minute nerve -fibres which are prolongations of 

 the nerve-cells. On the upper side of the nerve-ring are two 

 little swellings, that may be regarded as an incipient brain or 



chief central organ, and are technically 

 known as ganglia. A ganglion is a 

 thickened part of a nerve -cord, where 

 nerve-cells are concentrated as a result of 

 K evolution along centralizing lines. The 

 N.R. ventral cord swells into a pair of ganglia 

 in each segment of the trunk, for which 

 they act as controlling organs. In such 

 a form as the Earth-Worm the ganglia 

 are not very distinct, and nerve-cells are 



Fig. 1009. Front Part of the Central 

 Nervous System of an Earth-Worm, en- Scattered thrOUPTlOUt the WHOlC OI the 



, 1_ 1 r 



Central ttCrVOUS SyStCttl, DUt in the frCC- 



livin & Bristle- Worms and Leeches con- 

 centration of nerve-cells has taken place 



to a much greater extent, and the ganglia are clearly marked. 

 The relations of these active forms to their surroundings are com- 

 paratively complex ; hence greater concentration of nerve-cells with 

 increased efficiency of the nervous system. The complex nature 

 of the neurons will be gathered from fig. 1010, which represents 

 a few of them in part of the ventral cord of an Earth-Worm. 



The front end of a bilaterally symmetrical animal, such as a 

 segmented worm, is more subject to the action of external agents 

 than the rest of the body, and becomes specialized into a head, 

 in which the most important part of the nervous system, i.e. 

 the brain, and the chief organs of sense are located. Even in 

 a segmented worm we are justified in considering the brain as 

 the highest part of the nervous system, because it is the chief 

 centre of correlation and administration. Voluntary action, con- 

 sciousness or awareness of existence, sensation, and intelligence, 

 so far as these exist in so lowly an animal, are dependent upon 



larged. H.L., Head-lobe ; N.R., side of 

 nerve-ring; V. t visceral or sympathetic 



