The Circulatory System of the Cnidaria
Cnidarians are the one of the most simplest of organisms and they do not have a closed or open circulatory system,. Their circulatory system does not have any circulatory vessels. They mostly use diffusion as their means of circulating materials through their bodies. Diffusion is the most simplest form of circulating materials. The purpose of a circulatory system is to transport oxygen and nutrients, which the cells of that organism need, and remove waste. In a cnidaria, all the cells have contact with the water, which is full of oxygen and nutrients that the animal requires. Cnidarians have a very primitive gastrovascular cavity that utilizes diffusion to spread the needed materials and nutrients to other parts of the body. This gastrovascular cavity acts as a digestive cavity where diffusion and distribution of the required substances can occur. This cavity attempts to help the process of diffusion by reducing the distance a substance must diffuse to enter and leave the cell, through its large surface area in the organism. The animals contain two layer and a jelly-like mesoglea. Some cnidarians do contain three different systems: the nervous, reproductive, or the digestive.
Jellyfish
Jellyfish do not have a specialized circulatory system, but they use diffusion as a way to circulate materials throughout their body. Almost every cell is in contact with the environment and so they are in direct contact with the nutrients and materials they need. They do use diffusion though so that the materials circulate throughout the body. Also diffusion helps aid the second inner layer to receive all the nutrients, materials, and oxygen it needs. Jellyfish skin is thin enough for oxygenation to occur just by direct diffusion and because of this they do not require gills or lungs.
Coral
Coral does not have a circulatory system. Coral's stomachs mainly serve as the circulatory system because of its gastrovascular system. The many species of fish that live symbiotically with the coral also aid in the circulation of materials, oxygen, and nutrients for coral. In coral there is also a unique cell type, the cnidocyte, which helps to gather food. the food collected is taken into the central gastrovascular cavity where extracellular digestion takes place and then intracellular digestion. The branches of the gastrovascular cavity allow the materials, nutrients, and food to travel ot all parts of the coral.
Sea Anemone
Sea anemones "breath" and circulate oxygen through the main opening in the middle, which is used as a mouth. Once inside the oxygen, food, and materials are directly taken by the cells on the inside of the sea anemone. The cells on the outside absorb the oxygen, food, and materials directly from the water that they are in contact with. As with coral, sea anemones fish that symbiotically live with them which allows the coral to have aid in circulating all the materials needed. They have the same primitive "circulatory" system, which uses diffusion, as the jellyfish and coral have, except that sea anemones also have a main opening that could be considered a mouth.
- James Ikeda