NERVOUS SYSTEM 



143 



few in number are also subject to splitting into many fractions in 

 order to supply each muscle fibre within the thick longitudinal and 

 circular muscle blocks. Horridge (private communication) has, 



G. 



TL 



n 



u 



\! \! \! V- \! \! \r -^' 



V^=f\ 



y 



u 



fi 



V 



h 



n: 



IX 



c: 



?i 



S^ 



?v 



l\ (\ ^ 



E 



5^ 



Fig. 45. Diagram to illustrate four possible results of the con- 

 traction of circular muscles at one end of a cylindrical animal. In 

 A the muscles are all relaxed. In B the circular muscles of the right- 

 hand end have contracted and this end has elongated. The left-hand 

 end remains the same. In C the length of the right-hand end has 

 remained the same but the diameter of the left-hand end has in- 

 creased. In D the length of the right-hand end has also remained 

 the same. The length of the left-hand end has increased but not its 

 diameter. In E the length of both ends has increased but their dia- 

 meters have remained the same as in B and D (from Chapman, 1958). 



however, shown that in the polychaete, Harmathoe, the segmental 

 nerves contain very many neurones, distinguishable only with the 

 electron microscope, and the same state of affairs may be true of 

 Lumbricus. 



