The Sea Anemone's Cardiovascular system
The sea anemone also has a gastrovascular system, as do all species in the cnidaria phylum. Sea anemones also maintain a carnivorous diet, and use their tentacles to poison or stun prey, which is then funneled through their oral disk, or mouth. Because sea anemones have an adhesive base, they stay where they first sprouted and are able to filter feed for food as well. The thin and slimy exterior layer is used to absorb any needed nutrients and oxygen to be put into the gastrovascular cavity, which will distribute the materials throughout the body through absorption and diffusion. The cardiovascular system within the sea anemones is considered open because it absorbs needed materials for life, and then has it collect in the center of the organism where the materials bathe the internal structure of the organism. This uniform system is constant throughout all cnidarians because of a similar anatomy, as well as a circulation of life forms, from polyps, like anemones, to jellyfish, and back.
Here you can see the relation throughout the life cycle of cnidarians, and see as to why all of them share a very common structure, as they evolve from one form onto the next until it restarts. It is through the constant revitalization of life through a continuous cycle that this phylum carries on, unchanging, and exclusively is made of only these few species.