SENSE ORGANS AND BEHAVIOUR 



85 



of times that each leech settled in each zone is recorded graphically 

 in Fig. 46. This shows Glossiphonia heteroclita as the most strongly 

 photonegative in its behaviour. It also shows that blood-sucking 

 parasites of vertebrates settled more often in the lighted zone than 

 in either of the partially shaded ones. Apparently when these 



90 



80 



70 



60 



50 



40 



30 



20 



10 



H L 



D 



L 



T 



Fig. 46, The distribution of various species of leech between 

 (H) lighted, (L) lightly shaded, (T) heavily shaded and (D) dark 

 zones of an aquarium in a large number of trials, a, Piscicola 

 geometra; b, Hemiclepsis marginata; c, Theromyzon tessulatum; 

 dy Glossiphonia complanata; e, G. heteroclita ; /, Helobdella 

 stagnalis; g, Hirudo medicinalis; h, Haemopis sanguisuga; t, 

 Erpobdella. From Herter, 1936. 



Species are seeking a host they are photopositive, not merely 

 indifferent to light. By contrast, the two species of Glossiphonia 

 were found in lighted or partially shaded zones with about equal 

 frequency. 



Up to this point we have been concerned only with reactions to 

 diffuse lighting from above. Various people have also studied the 

 reactions of leeches to a beam of light from a single source. 

 Holmes (1905) gave a clear description of the behaviour of 



