The heart of a Daphnia can be seen through its body wall.

Using a microscope to view Daphnia collected from your local pond, you may actually be able to measure the heart rate on an individual animal. An assistant is helpful to time a ten second interval while you count the number of beats. Multiply the number of beats in ten seconds by 6 to get the beats/minute. It's a good idea to make several measurements (replicates) and then find an average heart rate.

As practice, you might try measuring the heart rate of the Daphnia heart viewed in this animation.