THE EARTHWORM AND ANNELIDS IN GENERAL 179 



considered the physiological unit of nervous activity. The ap- 

 paratus required for a simple reflex in the body of an earthworm 

 is represented in Figure 91. A primary sensory neuron (sc.), 

 lying at the surface of the body, sends a fiber (sf.) into the ventral 

 nerve cord where it branches out; these branches are in physio- 

 logical continuity with branches from a primary motor neuron 

 (me. ) lying in the ganglion of the ventral nerve cord. The second 



FIG. 91. Transverse section of the ventral nerve chain and surrounding 

 structures of an earthworm, cm., circular muscles ; ep., epidermis ; 

 1m., longitudinal muscles; me., motor cell body; mf., motor nerve 

 fiber; sc., sensory cell body; sf., sensory nerve fiber; vg., ventral gan- 

 glion. (From Parker in Pop. Sci. Monthly, modified after Retzius.) 



neuron (me.) sends fibers (mf.) into a reacting organ, which in this 

 case is a muscle. These fibers extending to the reacting organ are 

 called motor fibers (mf.) ; those leading to the ventral nerve cord 

 are termed sensory fibers (sf.). The first neuron, or receptor, re- 

 ceives the stimulus and produces the nerve impulse; the second 

 neuron, the adjuster, receives, directs, and modifies the impulse; 

 and the muscle or other organ stimulated to activity is the 

 effector. Within the ventral nerve cord are association neurons 

 whose fibers serve to connect structures within one ganglion or two 

 succeeding ganglia. These short neurons overlap one another, 

 and are doubtless responsible for the muscular waves which pass 

 from the anterior to the posterior end of the worm during loco- 

 motion. The three giant fibers, which lie in the dorsal part of 



