Phylum: Cnidaria
The Cnidaria phylum mostly consist of underwater creatures. The phylum is mostly made up by jellyfish, coral, anemones and sea pens. The Cnidaria phylum consist of over 10,000 organisms. Cnidaria have two types of body forms, swimming medusae and sessile polyps. The Cnidaria is a very unique group of organisms that stand out from the other groups.
Box Jellyfish
They are invertebrates, that lack a spine and even a brain.
But, even though they dwell underwater they also lack gills and lungs. Instead
of breathing like most organisms, they absorb it through a process known as
diffusion. These creatures are exposed to water at all times since it is
essential for their way of breathing. Since box jellyfish are always exposed to
water and they have a higher surface area then their body, they are able to
absorb the substantial amount of water. The box jelly fish carries special
cells around their body, which are able to capture water molecules. Oxygen is
used and carbon dioxide is released. By increasing their surface area ratio
they are able to take in more oxygen.
Coral
Coral is known to look like a rock with many colors, but this "rock" is alive. Like all things that are known to be alive, they have to have a way to breath. The Coral polyp has the ability to absorb water molecules with their surface. They also have the ability to modify the way they grow, this allows them to make an effective body structure that can take as much oxygen molecules in exchange for carbon dioxide.
Sea Anemones
Just like coral and the box jelly fish, sea anemones lack gills and lungs. But cells that cover the surface of sea anemones allow them to breath. The sea anemones takes in oxygen from water molecules and then releases carbon dioxide. The take in of water molecules relies on the waters current and their surface area.