ASSOCIATIONS WITH OTHER ANIMALS 103 



over 80 per individual Daphnia, hinder the movements of the water 

 flea and interfere with its feeding. Viaud's analysis of the associa- 

 tion showed that the rotifer is attracted to Daphnia by virtue of 

 the characteristic swimming movements made by the water flea. 

 An artificial water flea was made and was caused to move in water 

 in various ways; the numbers of rotifers settling on the artificial 

 host when it was moving in different ways was then recorded. 



CORONA 



eye 



LORICH 



FOOT 



TOBS 



Fig. 45. Brachionus rubens (Rotifera) from the surface of Daphnia 

 obtusa. The length of the lorica is about a quarter of a millimetre. 



First the number of oscillations per minute was studied, and it was 

 found that the greatest number of rotifers became attached when 

 the artificial host oscillated at a rate of just over a hundred per 

 minute. This is about the rate of the normal swimming movement 

 of Daphnia. The amplitude, or height of the oscillation, was then 

 studied, and again the greatest number of rotifers became attached 

 when the amplitude was near to that of a normal free-swimming 

 Daphnia. It is clear that the two components, frequency and 



