Explain Phylum Cnidaria - Coelenterates?
Members of the Phylum Cnidaria and one other group-Phylum Ctenophora (the comb jellies)-are the only two animal phyla that have radially symmetrical body forms. That is, their body parts are arranged in a pattern around an imaginary central axis. If a line were drawn through the axis, the parts appear to radiate in a symmetrical pattern, much like the spokes of a wheel.
Cnidarians are essentially carnivores, and they characteristically have soft gelatinous bodies that come in definite shapes. Cnidarians all possess tentacles that surround their mouths. These tentacles are used to seize prey and transport the food to their mouths. The tentacles bear cnidocytes, which are cells that specialize in stinging and capturing prey. The Phylum Cnidaria takes its name from these highly specialized stinging cells.
The stinging structure within each cnidocyte is a tiny harpoon called a nematocyst. Upon contact with prey, the nematocyst is discharged and shot out. Water, or hydrostatic, pressure propels the harpoon at considerable speeds-up to 2 meters per second! Nematocyst filaments are barbed, and they contain a toxic protein that stings and paralyzes prey. In fact, powerful neurotoxins from jellyfish are known to have caused the deaths of many swimmers over the years.
Cnidarians digest food much differently from the way sponges do it. The food drawn into the mouth by the tentacles enters the digestive cavity. There, cells that line the digestive (gastrovascular) cavity release enzymes that break down the food into smaller particles. These food particles can then be taken in by phagocytosis, and digestion is completed by the cells that line the gastrovascular cavity. Cnidarians were the first group of lower animals to evolve the ability to capture and digest prey larger than themselves. Being able to partake of a wide range of resources translates into greater feeding success, which is an evolutionary advantage.
Cnidarians have two basic body types. One type of Cnidarian forms a polyp body, and the other type forms a medusa body. A polyp looks like a vase with tentacles extending outward from the mouth, and a medusa is best described as a bell-shaped jellyfish that lives in the water column. Some polyps are colonial, while others live as individuals. Other Cnidarians have a life cycle that alternates between a polyp stage and a medusa stage.
Although the Cnidarians have more specialized tissues than sponges, their bodies are still relatively simple. They lack a true nervous system, but they do have a primitive nerve net. They lack muscle tissue, but are able to move by changing the shapes of their epidermal cells. Asexual reproduction takes place through budding, and sexual reproduction is carried out by the release of sexual gametes.
Most of the members of Cnidaria live in marine habitats, although a few species live in fresh water, such as Hydra. There are three major classes within the Phylum Cnidaria: Class Hydrozoa, Class Scyphozoa, and Class Anthozoa. Hydrozoans spend most of their life cycle in the polyp stage with a shortened medusa stage. Scyphozoans' life histories are opposite to those of hydrozoans. That is, they spend most of their life cycle in the medusa stage, with a short hydroid stage. Anthozoans ("flower animals") are made up of the corals and sea anemones. They are characterized by their flower-like appearance and hard external skeletons made of calcium carbonate.