78 LEECHES 



earthworms but not in leeches. Ventral stimulation by contact 

 with the substratum is necessary for normal locomotion in both 

 earthworms and leeches. The ventral nerve cord is capable of 

 co-ordinating crawling without the assistance of the head ganglia 

 but does not normally do so in the absence of peripheral stimula- 

 tion. Swimming by leeches is an activity for which there is no 

 parallel in earthworms, and the anal ganglion appears to be neces- 

 sary for normal swimming. Although locomotion in both forms 

 can be reduced to a pattern of reflexes there is no doubt that the 

 brain exerts a modifying influence on the pattern in response to 

 the stimulation of external conditions or internal physiological 

 needs. 



