Coral's cardiovascular system
Amongst coral populations, many polyps are born and grow of the coral heads, and so begin the cnidarian life cycle as the most fundamental unit. Coral protects itself through their calcium carbonate shell, which renders them motionless in the water. Coral, just like sea anemones, have a mouth, and are capable of eating small fish that wander close enough into the coral's branches and get poisoned. The main source of food for coral, however, is filter feeding and absorbing nutrients through its porous shell, and these nutrients are collected in the gastrovascular cavity, or the stomach, of the coral. The gastrovascular system is the only form of any cardiovascular, respiratory, or digestive system amongst the cnidarian phylum because it is what best suits them for their marine life and environments.