The sea urchin is different, having a different life strategy. Understanding differences helps us to see the necessity of having different organisms for different ecological niches in our environment. As an organism develops it often shows its phylogenetic heritage by the stages it goes through. See if, in your observations today, you can identify characteristics common to other phyla. Ask and answer questions while introducing the model to insure active participation and check understanding.
Students observe and draw dividing cells, blastula, gastrula, and plutei.
Tomorrow we start our own experiments using this system. Carefully note the Criteria for a Good Experiment on the overhead. Start thinking about what your group might do. This lab is a LOT OF SET up for the teacher, in terms of timing.