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 



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

 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, 

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



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

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

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

 ito the mouth. An arrangement quite similar to the receptor effector 

 Astern of the Coelenterates is found in the axon reflexes of mammals, in 

 which disturbances set up in certain sensory neurons spread directly through 

 collateral fibers to the vasomotor muscles of the skin (page 898). 



