Phylum: Echinodermata
Echinodermata is a phylum of invertebrate marine animals that can be found at any ocean depth. Echinoderms are recognized mostly because of their radial symmetry, having no right or left but top and bottom. There are about 7,000 living species in this phylum, making it the largest phylum without freshwater or land animals.
Starfish
Starfish are the most common echinoderms. Covering the body of a starfish are ambulacral feets, tubelike structure that allow them to move, and papulae, skin gills. These opening will allow the starfish to take in water. The water would then be used for oxygen. Any respiration waste will be excreted through the feet and papulae as well.
Sea Urchins
The body of a sea urchin is divided into 10 sections. Five of these sections are known as "ambulacral plates", which are covered with holes that tubelike feet would stick out from. These five sections are used for breathing. Instead of having a special organ or apparatus for breathing like most marine animals, sea urchins breathe through their ambulacral feet. Here water would be taken in, and then exchanged for oxygen.
Sand Dollars
Sand dollars, like starfish and sea urchins, have ambulacral feet. These feet do not only allow these echinoderms to eat and move, but they also play a key role in their respiratory system. Sand dollars would not take in water. Instead sand dollars would take in oxygen directly from the water through the thin skin of their ambulacral feet. This process is known as diffusion.