SENSE ORGANS AND BEHAVIOUR 



Table 5. Observations on the Tendency of Leeches to 

 Take up Their Position in a Crevice under a Glass Plate 



95 



former were much more positively thigmotactic. The way in 

 which hunger modifies this reaction is shown by Theromyzoriy 

 where the large animals with full gats were much more positively 

 thigmotactic than smaller specimens. 



Leeches are very sensitive to touch. As far as can be judged 

 from experiments with paint brushes and needles they are most 

 sensitive on the front of the head and least sensitive posteriorly. 

 Large or well fed specimens are less responsive than small or 

 hungry ones. When a Glossiphonia is resting in a shallow dish of 

 water and a needle is brought towards it it responds long before 

 the needle touches it. First there is a response when the needle 

 touches the surface film ; if it is performing undulatory movements 

 these cease abruptly or if it is resting then it presses its body closer 

 to the substratum. If the needle is now brought towards the 

 margin of the body this will be withdrawn before the needle 

 actually makes contact (Gee, 1912). In these experiments the 

 leech is presumably reacting to vibrations in the water caused by 

 the movement of the needle. ErpobdeUids respond to vibration 

 by rapid longitudinal contraction and parasitic forms may respond 

 by making searching movements or even by swimming upwards 

 in the water. 



Some leeches are able to locate the centre of a disturbance in 



