Cnidaria is a large phylum composed of some of the most beautiful of all the salt and freshwater organisms: the true jellyfish, box jellyfish, coral and sea anemones, and hydra. Although Cnidaria is an incredibly diverse group of animals, there are several traits that link them together.
Most cnidarians are dipoblastic, which means that they are composed of only two layers of cells. The outer layer is known as the ectoderm or epidermis, and the inner layer is known as the endoderm or gastrodermis. These layers contain the nerve nets that control the muscular and sensory functions of the animal. Between these layers is a jelly-like noncellular substance known as mesoglea, which in true jellyfish constitute the vast bulk of the animal (hence their common name). In other species, the mesoglea may be nearly absent. All cnidarians have a single opening into the body which acts as both the mouth and anus, taking in food and expelling waste. In most species the mouth is lined with tentacles which act to capture food. The mouth leads to a body cavity known as the coelenteron, where the food is digested. This body cavity has given this phylum its other, less commonly used, name of Coelenterata.
Cnidarians have a complex life cycle that, depending on the species, may alternate between two forms. The first form is known as a polyp, which is sessile (anchored to one spot). The polyps are tubular in shape, with the mouth, often lined with tentacles, facing upwards. The bodies often contain a type of skeleton that may surround the tissues (exoskeleton) or be surrounded by the tissues (endoskeleton). These skeletons may be composed of minerals like calcium carbonate, and/or may consist of organic material such as chitin. Polyps also have a hydrostatic skeleton, where the muscles in the endoderm work against the fluid contained in the coelenterons, thus extending the polyps. Hydrostatic skeletons are also present in the tentacles, allowing them to be extended to capture food. Polyps often form large colonies, where a trait known as polymorphism may occur: various polyps in the colony may take on specialized roles. For example, one polyp may only be used for defence, while another is used for reproduction and another for capturing food. Not all polyps do this, however, and may live solitary lives. Some cnidarians, such as true coral and sea anemones, live their entire lives in the polyp stage and do not metamorphose into the second form, which is known as the medusa.
In true jellyfish and in box jellyfish, the medusa is the most prominent form. They are free-floating or free-swimming, with the mesoglea giving them buoyancy. Medusae generally have only hydrostatic skeletons, which allow the muscles to work against the fluids in the coelenteron to enable the medusae to swim. The life cycle of cnidarians that contain both the polyp and medusa forms goes generally as follows: adult medusae reproduce sexually, creating a small, ciliated (cilia are...