From wikipedia

Daphnia are small, planktonic crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style (although fleas are insects and thus only very distantly related). They live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.

The division of the body into segments is nearly invisible. The head is fused, and is generally bent down towards the body with a visible notch separating the two. In most species the rest of the body is covered by a carapace, with a ventral gap in which the five or six pairs of legs lie. The most prominent features are the compound eyes, the second antennae, and a pair of abdominal setae. In many species, the carapace is translucent or nearly so and as a result they make excellent subjects for the microscope as one can observe the beating heart.

Even under relatively low power microscopy, the feeding mechanism can be observed, with immature young moving in the brood-pouch; moreover, the eye being moved by the ciliary muscles can be seen, as well as blood corpuscles being pumped around the circulatory system by the simple heart. The heart is at the top of the back, just behind the head, and the average heart rate is approximately 180 bpm under normal conditions. Daphnia, like many animals, are prone to alcohol intoxication, and make excellent subjects for studying the effects of the depressant on the nervous system – due to the translucent exoskeleton, and the visibly altered heart rate. They are tolerant of being observed live under a cover slip and appear to suffer no harm when returned to open water. This experiment can also be done using caffeine, nicotine or adrenaline and observing an increase in heart rate.

From Practicle Biology

"In the water flea Daphnia, the single, small heart is easily visible when viewed under transmitted light under a low power microscope. The heart rate (which can be up to 300 beats per minute) can be monitored and counted in different conditions – for example changing water temperature, or changing the type and concentration of chemicals added to the water. A change in Daphnia heart rate may not be a predictor of a similar change in human (or vertebrate) heart rate under the same conditions, but the procedure provides an interesting technique for investigating the effects of different chemicals on a metabolic process"

"Teachers should be careful to introduce these animals in a way that promotes a good ethical attitude towards them and not a simply instrumental one. Although they are simple organisms that may not ''suffer'' in the same way as higher animals, they still deserve respect. Animals should be returned promptly to the holding tank after being examined. This supports ethical approaches that are appropriate to field work where pond animals are returned to their habitat after observations have been made."