170 THE PHYSIOLOGY OF EARTHWORMS 



has centred on the three giant association fibres of the earthworm. 

 These are quick conducting pathways running the whole length of 

 the oligochaete body. Sensory input occurs into the median giant 

 anteriorly and to the lateral fibres posteriorly. They contain 

 septa which are not true synapses and conduction can proceed in 

 either direction across them although a functional polarity in the 

 animal is probable. Transmitter substances represented in the 

 central nervous system are acetylcholine, probably adrenaline and 

 noradrenaline and possibly 5HT. The properties of some sense 

 organs have been described. 



