EVOLUTION OF THE NEUROMUSCULAR MECHANISM 



829 



cles. The epithelial cell serves as a receptor for stimuli, and transmits 

 the disturbances set up by them to the muscle or effector. Such a mecha- 

 nism is called a receptor-effector system. The responses which it brings 

 about are purely local, since there is no provision for conducting the 

 disturbance to remote parts of the animal, but the introduction of the 

 receptor serves a valuable purpose in increasing the sensitivity of the 

 system. Moreover, the arrangement is adequate for the purpose for 



i> 



Fig. 203. The evolution of the nervous system. A, the independent effector as illustrated by 

 the muscle cells of the sponge; B, a receptor-effector system such as occurs in the tentacles of sea 

 anemones; C, the neuromuscular mechanism of the trunk of the sea anemone, in which a network 

 of nerve cells is interposed between receptor and effector; D, the nerve net surrounding a small 

 blood vessel of the frog; E, the nervous system of the earthworm, illustrating a typical reflex arc 

 and the occurrence of association neurons; a, within the ganglion; r, receptor; nn, nerve net; an, 

 afferent neuron; en, efferent neuron. (Modified after Parker, Prentiss and Bayliss.) 



which it is employed, which is to cause the tentacle to bend toward a 

 particle of food situated between it and the mouth, so that the currents 

 set up by the cilia with which the tentacle is covered may sweep the food 

 into the mouth. An arrangement not remotely analagous to the receptor 

 effector system of the Coelenterates is found in the case of man in the axon 

 reflexes by which local vasodilation is produced as the result of a local 

 irritation of the skin (page 898). 



