Fig. 1.1 shows an aquatic crustacean belonging to the genus Daphnia. These animals usually measure between 1-5mm in length. They have a two-chambered heart which can be seen through the exoskeleton when viewed using a low-powered microscope. heart [Figure 1.1] A biology teacher told a class that caffeine can temporarily relieve tiredness in people by increasing heart rate. Table 1.1 shows the caffeine content of various products. Table 1.1 product,caffeine per serving/mg 1 cup of strong coffee,100 50 g dark chocolate,45 1 caffeine tablet,50 1 can of cola drink,20 One group of students in the class used Daphnia to test the hypothesis: Heart rate increases when caffeine concentration increases.