Enchinodermata
- Phylum contains 7,000 species
- All species are marine animals
- Easily recognizable by the 5-point radial symmetry
- The three species are: starfish, sea urchins, and sea cucumbers
STARFISH
Starfish do not have a very well developed respiratory system. It is not clearly definable. They do, however, take in oxygen. It is through a rather simple manner. Starfish keep their internal liquids at the same salt concentration as the water around them. Because they essentially have salt water for blood, starfish simply take in the water around them through their tube feet, which are also used to suction themselves to surfaces, and the hard papulae that dot the body surface. Starfish do use simple gills to help with the movement of water in and out of the body cavity.
SEA URCHIN
Sea urchins have five sets of gills located around the mouth that they use to breathe. The gills are thin projections of the body cavity. These gills pick up water as the sea urchin moves or as the current hits the body of the sea urchin. This is when the gills collect the oxygen from the water and expel the carbon dioxide. As a backup plan in an incident of extremely low oxygen levels, a sea urchin can move muscles around the gills to help pumped water into them rather than just letting current and movement take care of it. A second backup plan ad the tube feet a sea urchin has. They are used as secondary organs of respiration behind the gills. They are used in the same manner that a starfish uses them.
SEA CUCUMBER
Sea cucumber have a somewhat well developed respiratory system. Like the starfish and the sea urchin, the sea cucumber extracts the oxygen from the sea water around them. This process is performed by a pair of "respiratory trees" that are located just inside the anus. So, the sea cucumber breathes by drawing water into the anus and then expelling it back out. The trees are made of thin and narrow tubules and the gas exchange occurs directly through the walls of these thin tubules to the liquid in the body cavity.