148 ORGANS AND ORGAN SYSTEMS 



The central nervous system is connected with the sensory 

 cells of the body wall, and all of the organs of the somites by 

 three pairs of nerves which leave the ganglia as shown in Fig. 

 60. These nerves are supported by the dissepiments and body 

 wall and branch and sub-branch until they are lost in a network 

 of fine fibers penetrating muscles and epithelium. Each of the 

 main nerves is a bundle of fine fibers which transmit sensory 

 impulses to the ganglia and motor impulses from the ganglia. 



At the anterior end of the ventral nerve chain there is a 

 pair of larger ganglia known as the sub-oesophageal ganglia. 

 The commissures from them pass around the oesophagus, one 

 on each side, and connect with a pair of ganglia in the peris- 

 tomium dorsal to the mouth. Because of their course around 

 the oesophagus these commissures are called the circum- 

 oesophageal commissures, and the two dorsal ganglia are called 

 the cerebral ganglia (c.c. and e.g.) This pair represents the only 

 morphological element comparable with a brain of higher ani- 

 mals, but it is probable that they have no functions different 

 from those of the ordinary ganglia of the ventral chain. As 

 there are more sensory cells in the anterior region of the 

 worm it is probable, however, that their functions are more 

 frequently called into play so that it is a more active organ 

 than any of the other ganglia. 



A Reflex Action. Consciousness, as we understand it in 

 human beings, probably does not exist in the earthworm, but 

 the relation between nervous impulse and muscular response 

 is so delicately adjusted that movements are produced which in 

 human beings we would interpret as conscious acts. The com- 

 plicated movements of a worm in its efforts to free itself from 

 some irritating environment, may all be traced back to a rela- 

 tively simple series of processes termed a reflex action (Fig. 

 61). Each reflex action involves five distinct elements of the 

 nervous system (i) A sensory cell which receives the stimulus 

 from the outside; (2) a nerve fiber bearing the sensory or 

 afferent impulse transmitted from the sensory cell; (3) a central 

 nerve cell in the ganglipn which receives the sensory impulse and 

 transforms it into a motor or efferent impulse which now travels 



