Hexactinellid sponges, also known as glass sponges, can be found all over the world, but are particularly populous in Antarctic Waters. These sponges stand upright to heights of up to 30 centimeters, forming strong bases that hold them to the ocean floor, with open central cavities that allow water to pass through. The glass sponge possess effective defense systems against microbes and parasites which involve engulfment of bacteria into specific cells, but also signal transduction pathways which actively kill bacteria. A signal transduction pathway is a set of chemical reactions in a cell that occurs when a molecule, such as a hormone, attaches to a receptor on the cell membrane. The pathway is actually a cascade of biochemical reactions inside the cell that eventually reach the target molecule or reaction. The engulfment of microbes and pathogens are like white blood cells in the human body.