244 



METAZOAN PHYLA 



are carried from the center to the effector cells is known as the efferent 



path. 



281. Reflex Acts.— In a typical and ideally simple reflex act a receptor 

 neuron situated in the hypodermis is stimulated from without (Fig. 152), 

 the stimulus being received directly by the cell body or by a short den- 

 drite. An afferent impulse follows the axon of this cell to a synapse in a 

 central ganglion where it is passed to the dendrite of a motor neuron. 

 This sends out an efferent impulse along its axon to a muscle cell, the 

 effector, causing contraction and resulting in a movement appropriate 

 to the stimulation which originated the act. This entire mechanism is 



Motor neuron 



Efferent nerve 

 fiber 



6/anf fibers 



venfrcrf ganglion 

 Synapse 



'Afferent 

 nerve fiber 



Sejnsory 

 n^urort 



Effectors 



Venfrai longitudinal 

 muscles 



Receptor 



Fig. 152. — Diagram illustrating reflex action in an earthworm. {From Parker, in Popular 



Sci. Mon., vol. 75, after Retzius.) 



called a reflex arc. Actually such a simple reflex does not occur, since 

 in any action several receptors are stimulated at the same time and several 

 effectors participate in the action. Also one or more connective neurons 

 are usually involved, and they form chains of neurons through which 

 conduction takes place. Since each of these connective neurons is in 

 communication with others, spreading of the impulse also occurs. 



In the earthworm not only may several connective neurons in the same 

 ganglion be involved in a reflex act, but it is also possible for impulses to 

 pass from one ganglion to another. This transmission of an impulse from 

 one metamere to another is due to connective neurons the axons of which 

 are known as association fibers. The association fibers are contained in 

 three tracts known as gf an/ fibers— though they are really bundles of fibers 

 —which he in the dorsal part of the ventral nerve cord. These fibers put 

 the cell of which they are a part into communication with cells in other 

 ganglia in front of the ganglion in which this cell is located or behind it. 

 In this way very strong stimuli may affect the entire body of the earth- 

 worm, causing it to act as a whole. 



282. Anterior Ganglia. — The two anterior pairs of ganglia of an earth- 

 worm are larger than the others, and to the first of these, or the supra- 



