FEEDING & DIGESTION

Cnidarians have two different life stages, polyps and medusa. Both have a body wall that surrounds the gastrovascular cavity (where digestion occurs). Cnidarians are predatory carnivores and will try to eat basically anything that comes into it's path. Most members of this phylum are filter-feeding relying on the current and water that flows through them.
Almost all cnidarians capture and eat small animals by using stinging structures called nematocysts located on their tentacles. Each nematocyst has a poison-filled sac containing a tightly coiled “dart”. When an animal touches a nematocyst, the “dart” uncoils and buries itself in the animal. The “dart” carries enough poison to paralyse or kill their prey. Once the prey is captured, the cnidarian’s tentacles push through the mouth and to the gastrovascular cavity to be gradually broken up. Special cells in the gastroderm (gastrovascular cavity lining) digest those tiny pieces of food. Nutrients are then transported throughout the body by diffusion.
In many cnidarians, tiny photosynthetic protists grow right in side the living cells of the gastroderm providing cnidarians nutrients through symbiotic nutrients. Photosynthetic protists use carbon dioxide and other wastes produced by the cnidarian’s cells to manufacture oxygen and organic compounds such as carbons and proteins. Protists use some of the oxygen and organic compounds themselves and release the rest into the tissues of their hosts (cnidarians). Also, some colonial cnidarians and jellyfish have long, tube shaped, branching gastrovascular cavities that help carry partially digested food through their bodies.