514 



THE EYE IN EVOLUTION 



of animal life, but at the same time they show a contractile response to 

 tactile stimuli as their oscula open and close with movements of the sea- 

 water. This is effected by the evolution of contractile myo -epithelial 

 CELLS, epithelial cells which acquire the power to contract when stimulated 

 and thus act as combined receptors and effectors ; in higher forms these 

 specialize in their contractile function and are displaced to form a muscular 

 layer beneath the epithelium (Figs. 671 and 672). In Parker's (1911-18) 

 view these muscular " independent effectors " are thus more primitive than 

 nerve cells. Nor, indeed, is this illogical, for since all primitive responses 



Figs. 673 to 678. — The Evolution of Receptors and Effectors. 



Fig. 673. 



Fig. 674. 



Fig. 675. 



n 



! 



Fig. 676. 



Fig. 677. 



Y Y 

 -o -o 



'k 



IV; 



X 



Fig. 678. 



Fig. 673. — A specialized sensory cell (sponge). 



Fig. 674. — A myo-epithelial cell (an independent receptor-effector) (jellyfish). 



Fig. 675. — A receptor-effector system consisting of a sensory cell with a 

 " nerve " fibril leading to a motor effector (muscle) (sea-anemone). 



Fig. 676. — The subepithelial nerve-net (jellyfish). 



Fig. 677. — An intercalary neurone between the receptor and effector (worms). 



Fig. 678. — A sensory organ, an afferent nerve (posterior root ganglion), an 

 intercalary neurone and an effector organ (Mammals). 



are tropisms, neither an independent receptor nor a conductor would be of 

 value were a muscular effector not available. Initially the sense-muscle 

 cell was stimulated directly ; only when the muscle became specialized 

 would specific receptors and conductors become necessary (Figs. 673 to 678). 



THE NERVE -NET 



In the Coelenterates, the first type of animal to require a wholly 

 coordinated body, a nervous system first made its appearance as a diffuse 

 nerve-net, 1 lying between the epithelial layer and the subepithelial muscular 

 layer. Into it dip down nerve-like processes from the sensitive epithelial 



^ For :]ic physiological mechanism of the nerve-net, see Romanes (1876-77), Bethe (1903), 

 Mayer (li 8), v. Uexkull (1909), Parker (1917-32), Pantin (1935-52), Prosser (general 

 review) (lUi ,.. 



