86 LEECHES 



Glossiphonia in such a situation. " In its progress the leech fre- 

 quently raises the extended anterior part of the body and waves it 

 from side to side as if feeling its way. If the animal turns it in the 

 direction of a strong light it is quickly withdrawn and extended 

 again, usually in another direction. If the light is less strong it 

 waves its head back and forth several times and sets it down away 

 from the light; then the caudal end is brought forward and the 

 anterior end extended and swayed about and set down still further 

 from the light than before. When the leech becomes negatively 

 oriented it may crawl away from the light, like the earthworm, in 

 a nearly straight line. The extension, withdrawal and swaying 

 about of the anterior end of the body enable the animal to locate 

 the direction of least stimulation and when that is found it begins 

 its regular movements of locomotion. Of a number of random 

 movements in all directions only those are followed up which 

 bring the animal out of the undesirable situation." 



The swaying of the anterior end of the leech, which Herter calls 

 Suchhewegung, searching movement, is very characteristic of 

 leeches which are crawling, or are about to do so. Gee said "This 

 tendency to 'prove all things, hold fast to that which is good' is 

 perhaps the most striking single characteristic of the behaviour of 

 leeches . . .". Herter showed that when Hemiclepsis is moving 

 towards a source of light it makes many searching movements and 

 ultimately turns and moves away. It then makes fewer searching 

 movements. The circumstances under which the leech was in- 

 duced to move towards the light were perhaps a little unfair. A 

 typical experiment is illustrated in Fig. 47. The leech was placed 

 in the centre of a circular dish with a 40 W lamp at one side. It 

 moved away on a path which made an angle of 35° with the direc- 

 tion of the light and eventually reached the side of the dish. It 

 then turned and moved along the side of the dish and in doing so 

 was induced to move towards the light. It made searching move- 

 ments on five occasions before turning back from the light. While 

 moving away from the light it made searching movements on only 

 one occasion. When the light source was changed to 150 W it 

 turned and retraced its steps while at a much greater distance from 

 the light. The unsatisfactory element in this experiment is that 

 the leech was almost certainly responding to the contact stimulus 



