Coral lack a central cardiovascular system, so they have a gastro-vascular system. Previously stated, this system combines the circulatory, digestive, and cardiovascular system. The nutrients and gases absorbed through the coral's outer cell layers are then diffused throughout its body. Although the coral do not move, they are able to feed off small fish swimming near them, and they are able to protect themselves since they have protective layers almost like a shell. When nutrients are absorbed, they are collected in the gastro-vascular cavity, which acts as a stomach for the coral. This combination of systems is essential in providing the coral what it needs to survive since it is immobile. Small algae and even fish, are able to live through mutual relationships with coral, and they are also given protection since the coral are large and do not move.