¶ … Taxa What is Cnidaria? What is it made up of? Who are the members that belong to his group? Cnidaria is an exclusively aquatic phylum. It is a group that is represented by the members, who are also called 'polyps', and these are sea anemones and corals, and also by 'medusae', which are creatures like the jellyfish. In general, both a polypoid as well as a medusoid cnidarian will be either radially or biradially symmetrical and it is an uncephalized animal with one single body opening, which is the mouth. Stinging capsules, which are embedded in the tentacles that are placed around the mouth, generally surrounds the mouth. These stinging capsules are also known as 'nematocysts', and these are capable of acting both as agents of defense as well as of offense. The most important distinguishing feature of a phylum is the presence of the intrinsic nematocysts, and in turn, nematocysts are the most widespread of the three different types of cnidae. (Cnidarians)

Cnidarians are diploblastic, or, in other words, they are made up of two cell layers, in the body, and in the tentacles, and they are the endoderm and the ectoderm. The endoderm is also known as the 'gastrodermis', while the ectoderm is also known as the 'epidermis'. The 'mesoglae' is found between these two cell layers, and this can be as minimal as a bit of binding element like glue, like in the case of a hydra, to hold the two layers together, or it can be as big as the body of the animal, like in the case of the jellyfish, whose body is made up of a single sac-like space, also known as the 'coelenteron'. This space communicates with its surrounding areas by way of its mouth, while the coelenteron serves the function of gas exchange and digestion. All cnidarians are carnivorous, and the tentacles being used for the purpose of capturing their prey. (Cnidarians)

Some of the better-known names of cnidarians are the Star Coral, or the 'Astrangia Danae', which is a variety of coral found in New Jersey. The 'Hydroid' or the 'hydractinia echinata', a polymorphic, is generally found on the shells of hermit crabs, and colonies of these creatures are found growing on rocks. The 'tubularia crocea' is another cnidarian, and it is also known as a mussel or algae, or a sea strawberry. The moon jelly is yet another cnidarian and it is also known as the 'Aurelia'. This creature has very small tentacles, weak nematocysts. However, it pays to be careful when handling these creatures because the venom that is present in their nematocysts may affect the mucous membranes of the human being. The 'Brown anemone' or the 'metridium senile' is another cnidarian, and this is an animal that boasts of possessing about 1000 tentacles, each armed with minuscule stinging threads. Its mouth is found at the very center of its shape, which is in the form of a disc. (Sandy Hook's Cnidarians) In the South Carolina Sea Islands, there are more varieties of cnidarians. Some of these are the sea squirts, and the sea oats or the Uniola. (South Carolina Sea Islands)

What are the unique features of cnidarians? One of the more important features possessed by this group of organisms is that they have nematocysts, or venom filled stinging cells. These stingers can be used by cnidarians either to capture their prey, or to defend themselves against any form of threat. These nematocysts are generally found on the inside of the digestive system of the creature. (Features unique to Cnidarians and to Ctenophores) These nematocysts come in about twenty-four different forms, and it is the basic difference between the nematocysts that play a major role in the classification of the phylum. Enclosed by a 'cnidocyst', the nematocysts are basically capsules like structures made up of something closely resembling a chitin, within which is found a coiled thread like structure. When the cnidarian closely encounters its prey, what it does is shoot out his thread. (The Phylum Cnidaria: www.earthlife.net)

The cnidocyst has either a flagellum in its modified state known as the 'cnidocil', or it has a small ocne that can be used as a sort of sensory trigger. Whenever this trigger is touched, the thread is immediately shot out of its casing, and the substance within is ejected. The threads of the nematocyst are classified into three types. The first one is a thin tubular thread with barbs at the far end, or at the near end, or at both the ends. When the nematocyst is ejected, the barb...

Another type of nematocyst is the 'ptychocyst', which is quite rare, occurring as it does in the Ceriantharians. These threads do not possess both spines and barbs; on the other hand, they possess adhesives, which they use to line the tubes, in which they live, and also to catch and to firmly entangle their prey. The third type, the 'spirocyst' also does not have barbs or spines; they are generally adhesive, and use this property to trap their prey and entangle them in the sticky threads. (The Phylum Cnidaria: www.earthlife.net)
Another unique feature that the cnidarians possess is the ability to regenerate, or the capability to grow back lost parts from their bodies. Some cnidarians are bioluminescent, or in other words, they have the ability to glow in the dark. This capability is used by the sea creature to demonstrate whatever it wants, as a sort of signal between the species. (Features unique to Cnidarians and to Ctenophores) Although the cnidarians appear to be extremely simple creatures, with a simple and uncomplicated life cycle, the reality is that these creatures are the most fascinating animals, because of the fact that they clearly demonstrate the complexity and the numerous steps that have been taken in the history of the evolution of life. (The Basics)

The Cnidarian is so unique a creature that it has been astounding scientists for ages by its methods of feeding and of reproduction; in fact, this small creature has been cloning itself for millions of years now. How does this creature feed itself? The hydra, a Cnidarian, is able to capture small and tiny crustaceans, and swallow as well as digest them inside its body, in its gut. The hydra uses two methods of digestion: one method is a primitive one wherein the food that it consumes is engulfed by the digestive system, after which it is digested in the numerous vacuoles that are located in the several phagocytic cells that line its gut. The second method of digestion that the cnidarian uses is that of 'extracellular' digestion wherein the entire digestive process is initiated by the hyphrolytic enzymes that are produced by the many glands that line the gut of the creature. When the process is completed, the nutrients present in the food are diffused on to the outer layers of the cells, even though there is no circulatory system present within the body. The process of regurgitation, wherein the food is sent out through the mouth, expels whatever food remains undigested. This may be one disadvantage of having a single ended gut system. (The Biology of Cnidarians)

The circle of tentacles that the cnidarian has around its mouth helps it to capture its prey, as well as to eat it. When the cnidarian uses its tentacles to capture its prey and entangles it among the threads of its nematocysts, then the prey that has been caught in the tentacles is moved to the mouth of the creature and then digested in the gastro vascular cavity, which is much like our own stomach, after which the nutrition is absorbed by the external cells. (Ocean link, all about the Ocean) Therefore, it can be stated that cnidarians have the simplest and most primitive form of digestive systems to be found in any living creature. The first point is that these simple creatures have only one single opening for the taking in of food, as well as for the elimination of wastes. When the food enters the mouth, certain digestive enzymes are secreted into the digestive cavity so that the consumed food can be broken up into smaller parts. Being carnivorous by nature, the cnidarian can eat small fish, and other small sea creatures. (Modes of Nutrition)

However, there are certain types of cnidarians that contain symbiotic dinoflagellate algae within the cells of their bodies. Also known as symbionts or zooxanthellae, these are responsible for producing energy rich food molecules for the body, after a process of photosynthesis, and some of this energy rich food leaks out to the surrounding cells of the cnidarians. There are some cnidarians that depend solely on this process for their survival, and if there were some loss of their zooxanthellae, then the creature would face death. Corals, including soft and stony, certain anemones, and certain jellyfish depend on zooxanthellae for survival. (About Corals, anemones, and their kin)

All species of cnidarians are in fact capable…