SENSE ORGANS AND BEHAVIOUR 89 



movements. In this way they are less easily seen by a predator. 

 The reflex involving cessation of ventilatory movements is one 

 which is particularly suited to experimental study. Common 

 European freshwater leeches which exhibit it are Hirudo, Haemopis 

 and Erpohdella. It is not necessary to shade the whole of the 

 animal, for under favourable circumstances they will respond to a 

 decrease in light intensity on a small part of the posterior sucker 

 (Kaiser, 1954). Decapitated leeches respond just as well as intact 

 animals, so clearly it is the epidermal light sensitive cells which 

 mediate the response. There is no response to shading the ventral 

 surface. 



If a leech is illuminated by two lights and one of them is switched 

 off, the fall in light intensity on the epidermis of a leech produces 

 the same effect as a shadow. By altering the intensities of the two 

 lamps it is possible to vary the percentage fall in light intensity. 

 Kaiser (1954) found that Haemopis responded to as little as 25% 

 decrease in light intensity. The suspension of activity did not last 

 indefinitely, ventilatory movements were resumed after 1 or 2 min. 

 If full illumination was then turned on, the experiment could be 

 repeated, and this time the period of inactivity was decidedly less. 

 With constant repetition of the experiment the period of inactivity 

 became shorter and shorter until eventually there was no response 

 at all to the stimulus. This habituation is illustrated in Fig. 48. 

 With 100% reduction in light intensity the first period of quies- 

 cence was 100 sec. It soon fell to about 10 sec, where it remained 

 for about 20 trials. After that the response quickly disappeared. 

 When the percentage reduction in light intensity w^as less the rate 

 of habituation was proportionately more rapid, so that at 30% drop 

 in light intensity the response disappeared after only three trials. 



Another reaction to light exhibited by many leeches is that of 

 colour change. It is brought about by pigment cells in the tissues 

 which change their appearance according to circumstances. They 

 are much branched cells in which the pigment may be either 

 concentrated into a small sphere in the centre of the cell, in which 

 case the cell is very inconspicuous, or may be dispersed throughout 

 the branches of the cell, making it conspicuous and contributing to 

 the general colour pattern of the animal. It is the type of chromato- 

 phore found, for instance, in Crustacea and vertebrates. The 



