PHYLUM ANNELIDA 



225 



called neurons. The neuron theory " supposes that there is no 

 nerve-fiber independent of nerve-cell and that the cell with all 

 its prolongations is a unit or a neuron ; that these units are not 

 united to one another anatomically, but act together physio- 

 logically by contact; that the entire nervous system consists 

 of superimposed neurons; . . ." (Barker.) 



The reflex carried out either consciously or unconsciously is 

 considered the physiological unit of nervous activity. The ap- 

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



"SL 



FIG. 157. Transverse section of the ventral nerve chain and surrounding 

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

 tudinal muscles; inc., motor cell body; inf.,- motor nerve-fiber; sc, sensory cell 

 body; sf., sensory nerve-fiber; vg., ventral ganglion. (From Parker in Pop. 

 Sci. Monthly, modified after Retzius.) 



is represented in Figure 157. A primary sensory neuron (sc.}, 

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

 tral nerve-cord, where it branches out; these branches are in 

 physiological continuity with branches from a primary motor 

 neuron (me.) lying in the ganglion of the ventral nerve-cord. 

 The second neuron (me.) sends fibers (mf.} into a reacting organ, 

 which in this case is a muscle. These fibers extending to the re- 

 acting organ are called motor fibers (mf.) ; those leading to the 

 ventral nerve-cord are termed sensory fibers (sf.}. The first 

 neuron, or receptor, receives the stimulus and produces the nerve 

 impulse; the second neuron, the adjiislor, receives, directs, and 

 modifies the impulse; and the muscle or other organ stimulated 



