100 LEECHES 



various other substances evoked normal locomotory exploratory 

 movements. Noxious substances caused the leeches to come to 

 rest with their anterior suckers out of the water but the rest of the 

 body still immersed. This suggests that the chemoreceptors are 

 confined to the head so that when this part of the body is out of 

 the water the leeches are no longer aware of the noxious stimulus. 

 In Table 6 is given a list of the substances tested and the reactions 

 they produced. Among the saturated fatty acids the most marked 

 effect was produced by formic acid. Sugars and alcohols produced 

 no effect in the concentrations used but alkaloids such as quinine 

 and caffeine gave rise to strong reactions. Substances which might 

 have been produced by a host animal, such as ammonia and urea, 

 gave rise to exploratory movements and substances which are 

 distasteful to man, such as indol and skatol, evoked a strong 

 reaction. 



The case quoted above of Hemiclepsis reacting to a glass rod 

 previously rubbed on the skin of a fish is an example of contact 

 chemoreception. Practically all the European freshwater leeches 

 react to a suitably prepared glass rod, Hirudo to one which has 

 been held under a man's armpit, Theromyzon to one which has 

 been in contact with the preen gland of a duck, or Helohdella to 

 one smeared with the blood of a snail. As long ago as 1916 Lohner 

 studied the reactions of Hirudo to various substances mixed with 

 its food. He persuaded the leeches to take blood by sucking at a 

 piece of skin stretched across a glass tube. While the leeches were 

 feeding he mixed the test substances with the blood in the tube 

 and noticed whether they caused the leeches to stop sucking. With 

 sodium chloride they reacted to a concentration of 7%, with 

 sucrose at 5% but with quinine at only 0-1%. It is therefore clear 

 that leeches have chemoreceptors which provide information 

 about substances in the water, substances with which the anterior 

 sucker comes in contact and substances contained in the blood 

 which is passing through the buccal cavity. 



