MUSCLE, NERVE AND LOCOMOTION 



71 



Ventral nerve cord 



Lateral segnnental 

 nerve 



Posterior dorso- lateral 

 cell capsule 



Fig. 38. Diagram illustrating the main routes taken by the 

 processes of the nerve cells lying in the ventral ganglion of a 

 leech. The four dorsal capsules are shown in firm outline, the 

 two median ventral ones in dotted outline. After Scriban, 1934, 

 based on the work of Retzius and Havet. 



same conclusion regarding the segmental nerves of earthworms. This 

 is an interesting difference between annelids and arthropods, for in 

 the latter there is multiple innervation of muscle fibres, providing 

 for both quick and slow response. The ventral nerve cord of leeches 

 contains a large number of fibres mostly extending over no more than 

 one segment ; it is perhaps not surprising that the normal conduc- 

 tion rate in this cord is rather low, of the order of 18 cm/sec in 

 Haemopis according to Schwab (1949), and 40 cm/sec in Hirudo 

 according to Lapicque and Veil (1925). Schwab found substantial 



