274 '^W^ ANIMAL KINGDOM 



given segment act together. All of the longitudinal muscles, or all of the 

 circular muscles, contract at a given moment. Independent movement of 

 the muscles on one side occurs only as the worm turns. 



121. Nereis and Lumbrkus: Nervous System 



The large bilobed brain is in the prostomium of Nereis, but mi- 

 grates posteriorly in the Lumbricus embryo to lie in the third segment. 

 Many small nerves extend to all parts of the anterior end of the body. 

 Paired circumpharyngeal commissures pass down around the anterior 

 end of the pharynx to join the subpharyngeal ganglion. This is also 

 bilobed; it is formed in Nereis by the ventral ganglia of the peristomium 

 (two fused segments), and in Lumbricus by a fusion of the ganglia of 

 the first three segments. The whole ventral nervous system arises as a 

 pair of longitudinal cords, but these fuse together to make an apparently 

 unpaired ventral cord. In each segment behind the peristomium the 

 cord thickens to form a ganglion, from which nerves emerge to supply 

 that segment. In most segments an additional pair of nerves passes for- 

 ward to the body wall of the next anterior segment. 



Locomotor activity, indeed all activities that pass in waves along 

 the body, are coordinated locally by the ventral ganglia. A series of 

 reflexes coordinate movements so that what happens in one segment 

 will occur a moment later in the next. This coordination is achieved 

 both by direct neural connections and by the tensions produced in one 

 segment by movement in the adjoining one. The entire system is so 

 constructed that an activity beginning at one end of the body will pass 

 automatically along its length. Hence adding more segments does not 

 noticeably increase the complexity of movement. 



Annelids may respond to an alarm with a sudden violent shortening 

 of the entire body. Both Nereis and Lumbricus keep the posterior end of 

 the body in their burrows as they forage, and this sudden shortening is 

 sufficient to pull the entire body back into the hole. Such a response 

 cannot be handled by the usual ventral nervous system with its numerous 

 ganglia and many synapses along the length of the body. Conduction is 

 very slow in this system; an impulse requires as much as 10 seconds to 

 travel the length of a worm 10 inches long. For the alarm response 

 annelids have giant axons, nerve fibers of large diameter that run the 

 length of the ventral cord. Nereis has three central fibers and a pair of 

 larger lateral fibers; Lumbricus has one very large central fiber and a 

 pair of smaller laterals. The speed of conduction along a nerve fiber has 

 been found to depend upon its diameter. These fibers are not only large, 

 but some of them extend the full length of the body without synapses. 

 Conduction along the giant fibers requires only a hundredth of a second 

 to travel 10 inches. In the earthworm, T. H. Bullock has fovmd that the 

 median fiber, which is the fastest, is activated by sensory information 

 from the first 40 segments of the body, whereas the lateral fibers respond 

 to sensations from segments posterior to this. 



Giant fibers are excellent material for physiological research, and 

 have been used extensively in studies of the nerve impulse. They are 



