I III EV0L1 TION OF Till M RVO STEM 





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



this possible it is n< Bsary that some means of communication become 



tabli&hed between* the differenl 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 passee 

 to the next or oven to some more distanl ganglion, where it ends by 

 synapsis. The important point to note is that tin--.. ;i-»hm;iT ion neurons 

 do not leave the central nervous system; they merely conned 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, 



-'03. — Schema of simple r< ; r, receptor in an epithelial memhrane; a, afferent fiber; s. 



synapsis; c, nerve cell of center; r , •; m, effector on 



which lies jusl above the gullet, and from which fibers pro< d 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 BCnse organs for sight, sound and 

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

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

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

 ture, to which the receptors of the integument an eially sensitised 

 They are distana receptors projicienl receptors . and it is evident that 

 the nerve reflexes with which they are concerned are of a higher 0] 

 than those located in the segmental ganglia themsel 1 



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

 the muscles of the head cud of the animal, but otic purely assoeia- 



