EVOLUTION OF THE NERVOUS SYSTEM 



contract and so withdraw the organism from any possible injury. I n 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 

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

 of the Ccelenterata, where we first find a definite nervous system. The object 



A B 



e.c. 



s.c 



m.p. 



m.c. 



FIG. 132. 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. 



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. 

 132, A), so that stimuli applied to the surface and acting on the epithelial 

 cells can cause, as an immediate response, a contraction 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 just mentioned, into special contractile structures. If in 

 the course of development the protoplasmic continuity between these two 

 sets of cells had not become interrupted (and we have no ground for assuming 

 that such an interruption occurs under normal circumstances), it is evident 

 that we should have so produced the simplest form of a reflex arc (Fig. 132, B), 

 namely, a sensory cell, which is stimulated by slight physical changes in its 

 surroundings and is thereby thrown into a state of activity similar to that 

 which we have already studied in muscle and nerve. This state of activitv 

 would be propagated by the protoplasmic channels to the muscular cell and 



