THE EVOLUTION OF THE NERVOUS SYSTEM 



785 



of the whole animal such as would be required for locomotion. To render 

 this possible it is necessary that some means of communication become es- 

 tablished between the different segmental ganglia. This is effected by 

 association neurons, each of which, as the name implies, consists of a nerve 

 cell with its dendrites located in one ganglion and of an axon, which passes 

 to the next or even to some more distant ganglion, where it ends by 

 synapsis. The important point to note is that these association neurons 

 do not leave the central nervous system; they merely connect various 

 ganglia. 



So far the ganglia of each segment are of equal importance, but if 

 we examine further we shall find that at the head end of the animal 

 several of the ganglia become fused together to form a larger ganglion, 



Fig. 203. Schema of simple reflex arc; r, receptor in an epithelial membrane; a, afferent fiber; s, 

 synapsis; c, nerve cell of center; e, efferent fiber; m, effector organ. 



Avhich lies just above the gullet, and from which fibers proceed around 

 the gullet to unite in front of it in another large ganglion, which usually 

 shows three lobes. These larger ganglia receive afferent nerve fibers 

 from the closely adjacent primitive sense organs for sight, sound and 

 smell, from structures, that is, that are really highly specialized recep- 

 tors. The cells of the retina and ear have been made capable of reacting 

 to impulses of light or sound instead of those of pain, touch or tempera- 

 ture, to which the receptors of the integument are especially sensitized. 

 They are distance receptors (projicient receptors), and it is evident that 

 the nerve reflexes with which they are concerned are of a higher order 

 than those located in the segmental ganglia themselves. 



Some of the neurons of the head ganglia are merely motor and act on 

 the muscles of the head end of the animal, but others are purely associa- 



