164 



THE PHYSIOLOGY OF EARTHWORMS 



fibre studied. Later microscopical studies showed that the nerve 

 sheath extended along the surfaces of these discontinuities. This 

 sheath material was thought to be myelinated and to have radially 

 orientated lipid molecules (Taylor, 1940). The presence of a definite 



Plasma 

 membrane 



Axon I 



Axon 2 



-*- Transmission 



Vesicles Myelin sheath 



c 



Fig. 52. Structure of intersegmental septum. A. after Hama, 1959, 



B. after Issidorides, 1957; de Robertis and Bennett, 1955. C. Septum 



of median giant fibre, set = septum, sc = small ganglion cells, 



neu = neurofibrils (from Ogawa, 1938). 



membrane structure was described after electron microscope studies 

 (de Robertis and Bennett, 1955 a, b; Issidorides, 1956) though 

 there are some disagreements over exact details. Numerous small 

 vesicles were figured on the pre-synaptic side of the septa, and in the 



