136 THE BIOLOGY OF AN ANIMAL. 



Nervous System. Co-ordinating Mechanism. By means of the 

 nervous system, the various parts of the body are co-ordinated 

 or made to work in harmony. Through its agency also the 

 worm receives impressions from the environment in the form of 

 sensations (sight, smell, touch, etc.) which enable it to act accord- 

 ing to the requirements of its daily life, or, in other words, to 

 maintain harmony with the environment. 



These functions are performed by slender cords of protoplasm, 

 the nerves, which receive stimuli and conduct impulses in an or- 

 derly manner from point to point through the whole organism ; 

 and by nerve-cells (centres), which receive, co-ordinate, and send 

 forth the impulses carried by the nerves. The nerve-cells are 

 arranged in small masses or ganglia, which, taken together, con- 

 stitute the central nervous system; the nerves collectively consti- 

 tute i\ie peripheral nervous system . 



1. Central Nervous System. This consists in the earthworm 

 of two parallel cords of nervous tissue running along the middle 

 line of the body on the ventral side, just inside the body-wall and 

 beneath the sub-intestinal blood-vessel (Fig. 80). Throughout 

 the greater part of the body the two cords are so closely united 

 as to appear like one, the double character only appearing in cross- 

 section (p. 160). At about the 4th somite, however, the two cords 

 separate completely, and embrace the front part of the pharynx. 

 Each cord in this region runs obliquely forwards and upwards at 

 the side of the pharynx and joins its fellow on the upper side, 

 forming a complete ring around the pharynx, the pharyngeal 

 collar (p.c\ Fig. 80). 



Behind this the double cord is usually called the ventral 

 nerve-chain. About the middle of each somite it is somewhat 

 enlarged to form a distinct ganglion (r.g, Fig. SO). The ventral 

 cord is, in fact, a chain of such ganglia connected by double cords 

 called commissures. 



Besides the ventral ganglia, there are two very distinct ganglia 

 lying side by side on the upper surface of the pharynx where the 

 two halves of the pharyngeal collar join (Fig. 80, <_:</). These are 

 the cerebral ganglia, so called because they are supposed to cor- 

 respond, in a general way, with the brain (cerebrum) of higher 

 animals. 



Besides these main parts of the central system, there are many .smaller 



