248 T. BRAILSFORD ROBERTSON. 



8. I have repeatedly confirmed the fact that the rate of the beat 

 at 21 is about 4.5, with a possible variation of about 5 per cent. 

 The organisms were all taken from the same vessel, in which 

 they were kept ; but they differed widely in size and develop- 

 ment, some containing eggs and others containing well-developed 

 embryos. Ceriodaphnia therefore affords homogeneous material 

 for experiments on the heart-beat. 



