88 LEECHES 



from the side of the dish. It will be shown on p. 94 that leeches 

 are strongly thigmotactic. If it had not been for this complicating 

 factor the leech would probably never have moved towards the 

 light for any considerable distance. 



Herter found that none of the leeches which he studied moved 

 directly away from the source of light when free to do so. Piscicola 

 moved along a path which was up to 48° from the line of the light 

 rays and Hemiclepsis up to 90°. On the other hand Gee found that 

 Helohdella frequently moved almost directly away from the light 

 source, pausing to make frequent searching movements as it did so. 

 Both these workers concluded that there is a very considerable 

 degree of random movement in the responses of leeches to light 

 and Herter found this surprising in view of the level of complexity 

 of the leech eye. Since he worked mainly with Glossiphoniidae, 

 which have the eyes rather far below the surface of the head, it 

 would be interesting to investigate the behaviour of leeches with 

 more superficial and more complex eyes, and to study the effects of 

 unilateral blinding and of varying the direction of the incident 

 light during the course of an experiment. 



Leeches have no statocysts or other organs for orientation in 

 relation to gravity and it appears that when swimming they make 

 use of the fact that light normally falls on them from above and 

 exhibit a dorsal light reaction. If a swimming medicinal leech is 

 suddenly subjected to light from below and not from above it will 

 swim in a vertical arc until its dorsal side is directed towards the 

 light (Schluter, 1933). It does not maintain this position in- 

 definitely, however, but eventually returns to its normal orienta- 

 tion, presumably in response to the effect of gravity on the internal 

 organs. A similar dorsal light reaction is shown by leeches placed 

 between two glass plates and this response is shown even by 

 decapitated leeches, indicating that the eyes are not essential to 

 the reaction, the light sensitive cells of the dorsal body wall being 

 sufficient. 



When a shadow passes over a leech in its natural habitat it is 

 likely that this is caused by the movement of a larger animal. The 

 blood-sucking parasites often react by making searching move- 

 ments, or even by swimming upwards through the water. The 

 non-parasitic forms react in other ways; they may flatten them- 

 selves against the substratum or abruptly cease making ventilatory 



