The ultimate function of most physiological processes is to help organisms maintain overall homeostasis, while letting them adapt to changes in internal and external environments. In this unit, heart rate is the model for studying homeostasis. The students use aquatic blackworms, and as their name implies, they live in slow-moving or stagnant freshwater. The instructor shows students the dorsal aorta of a blackworm and how to prepare center segments of worms for observation. Once students identify the heart and prepare the worms, they will complete an exercise looking at how neurotransmitters and other chemicals affect heart rate. Students then design an experiment of their own to test one or more physiologic responses of blackworms and at the end of the unit students perform the experiment.