8 NERVOUS SYSTEM AND SENSE-ORGANS 
side 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 
evolution along centralizing lines. The 
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 
Fig. 1009.—Front Part of the Central’ 2° NOt very distinct, and nerve-cells are 
Nervous System of an Earth-Worm,en- Scattered throughout the whole of the 
larged. H.L., Head-lobe; WV.R., side of 2 
nerve-ring: V7 visceral or sympatheic Central nervous system, but in the free- 
aie Siete ais living 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, /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 
