SENSE ORGANS AND BEHAVIOUR 97 



and Hirudo medicinalis congregated near the surface of the water. 

 Presumably the leeches detect the force of gravity from its effect 

 on the tissues of the body. 



When light and gravity responses are in conflict it is the light 

 response that overrides the other on most occasions. When the 

 upper half of an aquarium was shaded while the lower half was 

 lighted specimens of Hirudo came to rest in the upper half on 

 89% of the occasions observed but when light and shade were 

 reversed they came to rest in the lower half on 76% of the occasions. 

 Another factor modifying geotaxis is oxygen lack, for many leeches 

 which would otherwise remain near the bottom rise to the surface 

 when oxygen is scarce. 



6. Reactions to Chemical Substances 



It is recorded that Hemiclepsis can with difficulty be persuaded 

 to suck blood from Raiia temporaria, that it will never suck the 

 blood of Rana esculenta but that it sucks readily on the larvae of 

 Pelohates. The basis of such discrimination may be partly tactile 

 but is almost certainly mainly chemical. Again, Hemiclepsis will 

 often ignore a glass rod brought into contact with its head but if 

 this glass rod has been rubbed on the skin of a fish it will attach 

 itself with alacrity and attempt to suck from it. There is therefore 

 no doubt that leeches have a well developed chemical sense. 



Some authors have distinguished between taste and smell in 

 leeches. By this they have meant the distinction between a 

 response to chemical substances drifting in the water and a 

 response to close contact between the surface of the animal and a 

 particular substance. In fact there is no evidence at all that leeches 

 have more than one kind of chemoreceptor and it is perhaps best 

 to regard the two sensations as two aspects of the one activity. 



Examples of response to substances drifting in the water are 

 afforded by the reactions of various leeches to extracts of their 

 normal host animals. In most cases the reaction is random and 

 undirected ; for example, if the juice of a pond snail is added to the 

 water in a dish containing Glossiphonia complanata the leeches may 

 respond by making searching movements, especially if their guts 

 are empty, but their movements do not bring them to rest at the 



