70 



LEECHES 



section (Fig. 37) passes through two dorsal and one ventral capsule 

 and shows the nerve cells as pear-shaped structures with their 

 narrow ends directed towards the centre of the ganglion. They 



Ventra 



Transverse and 

 longitudinal fibres 



Neuroglia cell 



Nerve cell in 

 dorsal capsule 



Neurilemma 



Giant ceil in 

 ventral capsule 



0-1 mm 



Fig. 37. Transverse section through a ventral ganglion of 



Haemopis. 



give off processes which pass through the capsule wall and join the 

 fibrous mass. The course of the axons was studied by Retzius 

 (1891) and by Havet (1900) and their work has not been improved 

 upon. The main motor axons run from a given cell capsule into 

 the lateral segmental nerves of their own or the opposite side of 

 the body. Internuncial neurones form synaptic connexions with 

 both sensory and motor fibres, often running from one ganglion 

 to the next along the ventral cord. In addition to these there is 

 a giant cell in each ventral capsule and this has an axon which 

 forks, sending one process into the ventral cord and two into 

 lateral segmental nerves. There are no giant fibres of the type 

 seen in earthworms and polychaetes, but from the behaviour of 

 leeches it is clear that there is a fast conducting system (Miller, 

 1942). From Retzius' figures the axons of the ''giant nerve cells" 

 appear to be about twice the diameter of a normal nerve fibre. 

 It is doubtful whether these fibres would mediate the startle 

 reaction of leeches unless they were also myelinated. Wilson (1960) 

 studied the effect of stimulating an isolated segmental nerve of 

 Hirudo and found no evidence of a quick response in the peripheral 

 neuromuscular system. Horridge and Roberts (1960) reached the 



