EARTHWORM 97 



motor neuron with the cell bodies present in the ventral nerve 

 cord, and the axons extending peripherally for considerable dis- 

 tances to innervate the muscle tissue in the body wall and alimen- 

 tary canal. A second type is the sensory neuron with the cell 

 bodies situated in the various tissues and organs all over the body, 

 but particularly in the epithelial layer where they are in close 

 contact with the environment. The axons from the sensory neu- 

 rons run to the ventral nerve cord. A third important type, the 

 adjustor neuron, is situated entirely within the nerve cord 

 where contact (synapse) is made with the axons from the sensory 

 cells and with the dendrites of the motor cells. The segmental, 

 peripheral nerve trunks consist of many axons insulated from each 

 other and bound together with supporting tissues to form the 

 definitive nerve. Each nerve contains both motor and sensory 

 axons. (W. f. 138.) 



It is this differentiation of the neurons that makes it possible to 

 bring about harmonious action in response to a stimulus. For 

 example, if the skin of a living Earthworm is touched with a 

 dissecting needle, coordinated movements of the body will follow 

 almost immediately. In such a case the sensory neurons in the 

 peripheral regions receive the external stimulus, and a nerve im- 

 pulse is transmitted from them over their sensory axons to the 

 central nervous system. Here in the cord the message is trans- 

 ferred to the adjustor neurons which, in turn, refer it to the 

 proper motor neurons whose axons innervate the muscle fibers in 

 the affected region. The muscles then contract in accordance 

 with the stimulus originally received by the sensory cells. Such a 

 circuit is known as a reflex arc, and the resulting action as a 



REFLEX ACTION. 



Sense Organs. The sensory neurons in various regions of the 

 body may be specialized for the reception of a particular type or 

 types of environmental stimuli, and also grouped together to form 

 definite sense organs of essentially the same nature as in the 

 higher Vertebrate animals. Although the sense organs of the 

 Earthworm are microscopic in size they nevertheless exhibit 

 remarkable differentiation. Thus for receiving stimuli from light 

 waves there are modified sensory neurons, or photoreceptor 

 cells, present in the skin epidermis and grouped at intervals 

 along certain nerves. They are particularly abundant in the 

 prostomial region and at the extreme posterior end of the body. 



