96 



LEECHES 



the water. Figure 50 illustrates an experiment in which Theromyzon 

 reacted to a disturbance created at three different points in succes- 

 sion by blowing down a tube on to the surface of the water. It is 

 obvious that it was able to direct its movements towards the 

 source of disturbance with a fair degree of accuracy. It is also 



Fig. 50. The path taken by Theromyzon when a disturbance 



was created successively at 1, 2, and 3 by blowing down a tube 



on to the surface of the water. From Herter, 1929. 



well known that Hirudo uses this method to locate its host, for 

 hungry leeches will converge on a centre of disturbance in a pond 

 in which they are living even if the disturbance is made by moving 

 a stick rather than by any part of an animal. 



5. Reactions to Gravity 



Leeches have no specific organs of gravity perception yet most 

 of them show a positive geotaxis as was shown by Herter's 

 observations on the distribution of various species in a tall glass 

 tube, precautions being taken to eliminate light influence. He 

 found that Helohdella stagnalis, Glossiphonia complanata, G. hetero- 

 clita and P. geometra were resting in the lower 10 cm of the tube 

 on more than 90% of the occasions while Hemiclepsis marginata 

 and Erpohdella sp. showed a greater tendency to wander upwards, 

 being found in the lowest 10 cm on just over 70% of occasions. 

 On the other hand, hungry specimens of Theromyzon tessulatum 



