76 LEECHES 



does so when stimulated from the peripheral nervous system or 

 the ganglionic masses. 



Clearly the reflex pattern described by Gray et al. is only part 

 of the mechanism of locomotion, for there are many exceptions to 

 the pattern they described. When a leech changes from crawling 

 to swimming it does so in spite of the ventral peripheral stimulation 

 which it is receiving. When it performs ventilatory movements it 

 in effect swims while retaining a hold with the ventral sucker 

 (Fig. 42). It is likely that the large ganglionic masses of the head 



Fig. 42. Lateral view of Erpobdelia with ventilatory movements 



in progress. 



and anal regions play a part in determining locomotory behaviour. 

 Kaiser (1954) has considered in some detail the experimental 

 evidence for ganglionic activity. Decapitated leeches swim more 

 readily and for longer periods than intact ones. This is partly due, 

 no doubt, to the fact that the ventral nerve cord no longer receives 

 impulses from the anterior sucker, as these would normally 

 initiate crawling. The swimming movements of a suspended leech 

 may be stopped by stroking the ventral surface and conversely 

 they may be accentuated by gentle dorsal stimulation. However, 

 if the cerebral ganglion is removed while the suboesophaegeal is 

 left intact the leech is still abnormally active. It appears that one 

 of the functions of the cerebral ganglion is to inhibit locomotion 

 under certain circumstances. Another function, which we may 

 deduce from a study of the pathways of nerve fibres, is to receive 

 and sift information from the sense organs of the head. 



Removal of the sub-oesophageal ganglion brings about a number 

 of changes at one time. It isolates the ventral nerve cord from the 

 influence of the brain, isolates it from peripheral stimulation 

 through the anterior sucker and removes the influence of the sub- 

 oesophageal ganglion. Leeches treated thus appear to be almost 

 incapable of crawling, remaining inert for long periods. Budden- 

 brock (1953) has pointed out that a number of experimental results 



