EVOLUTION OF THE NERVOUS SYSTEM 



325 



peculiarly nervous. A stimulus applied to any part of the amoeba 

 may evoke responsive activity in all other parts. A slight touch 

 applied to any point on a vorticella will cause an excitation which is 

 rapidly propagated to the stalk, causing this to contract and so 

 withdraw the organism from any possible injury. In the lowest 

 metazoa, such as the sponges, we find no special nervous structures. 

 The cells forming the sponge may react to changes in their 

 environment by contraction or by alteration of their relative 

 positions. Many of the cells can move from one part of the 

 sponge to the other in response to chemical changes occurring 

 in the body of the sponge. So far, however, no cells have been 

 distinguished as endowed above their fellows with the property of 



m.p. 



m.c. 



FIG. 133. Diagrammatic representation of evolution of a nervous system. 



(Modified from FOSTER.) 



ec, epithelial cell ; mp, muscular process ; sc, sensory cell ; np, nerve 

 process or fibre ; me, muscle-cell ; snp, sensory nerve process ; mnp, motor 

 nerve process ; cc, central cell. 9 



irritability or the power of reaction to stimulus. It is in the next 

 class, that of the Coelenterata, where we first find a definite nervous 

 system. The object of a nervous system is to ensure the co-operation 

 of the whole organism in any reaction to changes in its surroundings. 

 At its first appearance therefore we should expect a nervous 

 system to be developed in connection with that layer of the animal 

 which is in immediate relation to the environment, namely, the 

 epiblast or external layer. In some species of hydra, though no typical 

 nervous tissues have been detected, many of the epithelial cells lying 

 on the surface are prolonged at their inner ends into a long contractile 

 process (Fig. 133, A), so that stimuli applied to the surface and acting 

 on the epithelial cells can cause, as an immediate response, a con- 

 traction of the underlying muscular processes. We may easily 

 conceive that in such an animal, among the cells forming the epiblast, 

 certain cells might become endowed with a special sensitiveness to 

 external changes, other cells being developed, like those of the hydra 



