SENSE ORGANS AND BEHAVIOUR 91 



circumstances. Glossiphofiia complanata which is not of this habit 

 does possess variable chromatophores but they are so sparsely 

 scattered in the tissues that they make little difference to the 

 appearance of the animal (Wells, 1932). 



Kowalewski (1900) claimed that Placohdella costata, a parasite 

 of the European water tortoise, was green when resting on vegeta- 

 tion but brown when on a tortoise. This suggests that the leech 

 possesses at least a rudimentary form of colour vision. Denzer- 

 Melbrandt (1935) studied the question of colour vision in leeches. 

 She prepared test containers in which the incident light passed 

 through various shades of coloured paper, and she observed in a 

 large number of trials which colours the leeches preferred. She 

 claimed that Helobdella and Hirudo showed a preference for colours 

 of longer wavelength, even when this involved moving into regions 

 of higher light intensity. The method used for determining the 

 light intensity involved making comparisons by the human eye. 

 If the leech eyes happened to be less sensitive than the human eyes 

 at the red end of the spectrum it is possible that they were in fact 

 showing a preference for the zones which appeared to them to be 

 illuminated at a lower intensity. We must therefore regard the case 

 for the existence of colour vision in leeches as not proven. 



On the other hand. Smith (1942) showed clearly that the 

 chromatophores oi Placohdella parasitica respond to simple changes 

 in light intensity. This leech has three kinds of chromatophores, 

 yellow, green and reddish brown. Of these the green are most 

 active in colour change, expanding in light and contracting in 

 darkness. It appears that the chromatophores are under nervous 

 control, for electrical stimulation at either end of the body will 

 cause a pale leech to darken, but if the nerve cord is transected the 

 effect will not be produced beyond the cut. Decapitation causes 

 a pale leech to darken, but after it has recovered from the shock 

 of the operation it will still change colour slowly in response to 

 changes in light intensity. This shows that the eyes are not essen- 

 tial to the process and that the light sensitive cells of the epidermis 

 can mediate the response. In fact, if a decapitated leech is brought 

 from darkness into the light and one part of the body is shaded 

 that part will remain paler. Vavrouskova (1952) studied the colour 

 change mechanism of Theromyzon tessulatum and concluded that 



