ORIGIN OF THE NERVOUS SYSTEM 14 



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which the eye commences as a mere open pit, then 

 develops into a closed vesicle, and passes by degrees 

 into an organ whose complexity approaches, if it does 

 not attain, that of our own ; whilst with regard to the 

 ear, the skates and certain other fish still retain an 

 open tube by which their ear is placed in communica- 

 tion with the skin. 



In the early life of animals the nervous system 

 itself is at first in close contact with the embryonic 

 skin, and only gradually sinks down to take up its 

 position in the enclosing bony or gristly tube that is 

 to form the vertebral column ; whilst in many primitive 

 forms its connection with the skin is retained through- 

 out life. The nerves which direct movement are 

 actually developed in connection with the muscles 

 and skin, much as in the case of a jellyfish or sea- 

 urchin ; whilst those nerves that announce hunger, 

 desire, and generally the condition and needs of the 

 inner organs, are derived ultimately from ingrowths 

 of the surface layer. The two sets of organs — the outer 

 or directive and inner or visceral — become inter-con- 

 nected by nervous ties that provide pathways whereby 

 the need for food or breath, for example, is able to set 

 in motion a movement of the body that tends to supply 

 that need. So we see a full animal is quiescent, 

 a hungry one is active. In the reverse way, the sight 

 or taste of food sets going the nerves that connect 

 the prehensile organs with the digestive ones, and so 

 prepare the way for the reception of food. So short- 

 ness of breath induces more rapid breathing, as we see 



