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ANIMAL LIFE 



emphasised in the primitive nervous systems, such as 

 those which jellyfish and starfish possess. In fact, the 

 development of these animals shows that the muscle 

 and skin are at first continuous, and the strained 

 threads that connect the two, as they separate for 

 their respective functions, become the nerves for em- 

 phasising movement. In a similar way the food-tube 

 is at first a part of the skin, which becomes pitted in 

 and grows inwardly, and the connection between its 

 layers and the muscles and skin of the wall of the body 

 furnishes the basis for its visceral nerves, and for the 

 correlation between their demands and the exertions of 

 the locomotor organs in the prehension and supply of 

 food. Thus the nervous system of animals is inti- 

 mately related to the primary conditions and needs 

 of life, and is organically descended from the neuro- 

 muscular skin and the neuro-muscular food-tube. 



The development of animals, even of ourselves, 

 shows how close is the relation of the nervous 

 system to this primitive plan just sketched, and how 

 intimately the sense-organs are connected with the 

 skin. The skm in all animals is an organ of one or 

 more senses, and from it arise the essential sensitive 

 parts of those sense-organs, such as the eye, the ear, 

 and nose, which seem to be unrelated to it. It would 

 appear incredible that the retina of the eye, the equally 

 delicate and more complicated lining of the ear, should 

 be a modified patch of skin ; yet such is the case, not 

 only in one group, but in all ; and, indeed, in some 

 groups it is possible to poinLuut a series of stages in 



