340 



CHORDATE ANATOMY 



connexions that, if all were destroyed except the nervous tissues, the 

 general form of the body would still be preserved. 



The steps in the evolution of a reflex nervous system such as that of 

 worms and vertebrates involve, first, the differentiation of a neurosensory 

 cell in the ectoderm. The body of such a cell remains in the ectoderm, 

 and one or more protoplasmic hairs may extend above the surface. The 

 most characteristic feature of such a cell, however, is the neurite which 

 grows away from the surface towards the underlying muscles. By branch- 

 ing into terminal telodendria, such a neurite may increase the number of 

 its connexions. 



EPITHELIAL CELL 



MUSCLE CELL 



TOioi4^ioioH=iirTnwniFE 



--SENSOIW CELL- 



miMoioKi-iiiiiiiTrrn 



METAMERIC MUSCLES 



Fig. 301. — A diagram showing hypothetical stages in the evolution of the reflex arc of 

 the higher animals. In A the series begins with the neuromuscular cell of coelenterates. 

 In B the neurosensory cell becomes differentiated from the muscle fiber. In C the 

 body of the neurosensory cell recedes from the surface. In D the neurosensory cell 

 becomes a sensory neuron and a secondary sensory cell conveys external stimuli to the 

 sensory neuron. In £ a motor neuron is interpolated between the sensory neuron and 

 the muscle fiber. Finally, in F, by means of several motor neurons, the connexion of a 

 sensory neuron with several muscle fibers is effected. (Redrawn after Fritz Kahn, 

 "Der Mensch," Albert Miiller. Zurich.) 



A second evolutionary step is taken when the body of the neurosensory 

 cell sinks below the surface into the underlying connective tissue, but 

 retains connexion with the superficial epithelium by means of a process 

 with branched terminations or dendrites attached to the skin. 



Further advance appears when the dendrites, instead of ending freely 

 among the epithelial cells, become connected with special receptor or 

 secondary sense cells in the skin. The somatic sensory cells of verte- 

 brates are at this evolutionary stage. 



The so-called primitive ganglion cells of coelenterates exhibit another 

 line of differentiation. That these cells derive from neurosensory cells 

 is a conclusion supported by considerable evidence. That they are more 

 differentiated than neurosensory cells is indicated by the fact that they 



