In this lesson, students will observe and count the heart rate of Lumbriculus worms with and without the addition of a plant extract. They will learn about how chemical transmission works in the nervous system and circulatory system, using two plants as case studies: foxglove and valerian. In the lab portion, students will explore how plant extracts act as stimulants or depressants and affect the circulatory system of Lumbriculus. Because their circulatory system can be easily viewed, Lumbriculus are a widely used model organism.