Did You Know? Corals, that are polyps belonging to Phylum Cnidaria, secrete calcium carbonate to form coral reefs. If all the coral reefs of the world were brought together in one place, the singular mass would span an area twice the size of the United Kingdom!

Members of phylum Cnidaria are distinguished and classified on the basis of their body form. They occur either in a swimming medusae form or a sessile (permanently attached) polyp form. They are classified into four classes: Anthozoa, Scyphozoa, Hydrozoa, and Cubozoa. The medusa form is characterized by an umbrella-shaped organism that has a fringe of tentacles, a mouth cavity on the lower-facing side of the bell, and a gastrovascular cavity. This form is most commonly associated with adult jellyfish. The polyp is a structure which has a mouth opening at the top, numerous tentacles springing out from the edge of the mouth opening, and a cylindrical body stalk which is attached to a substrate at the base. The gastrovascular cavity is within the cylindrical body stalk. This form is most commonly associated with sea anemones, corals, and hydras. The Cnidarians from class Anthozoa are characterized by a floral appearance. This class, therefore, gets its name from the Greek words ánthos (flower) and zóa (animals). Another major characteristic of these species is that they are sessile, that is, they are permanently attached to a substrate at the base, and cannot move about freely. They exist only in the polyp form. Giant Green Anemone (Anthopleura xanthogrammica), Sea pen (Ptilosarcus gurneyi), Deepwater sea fan (Iciligorgia schrammi), and Pulsating xenid (Heteroxenia fuscescens) are some examples of Cnidarians that come under Class Anthozoa. Species within Class Scyphozoa are distinguished from others due to their drinking-cup-like appearance. Hence, they get their name from the Greek word skyphos, which means a deep wine drinking cup on a low-flanged base or none. Unlike Anthozoans, species of class Scyphozoa are free-moving and not sessile. They exist only in the medusa form throughout their entire life cycle. Jellyfish belong to Class Scyphozoa, which is why, this class is also sometimes known as the 'true jellyfish'. Some examples are Lion's mane jellyfish (Cyanea capillata), Flower hat jelly (Olindias formosa), Barrel jellyfish (Rhizostoma pulmo), Moon jelly (Aurelia aurita), and Compass jellyfish (Chrysaora hysoscella). Hydrozoan Cnidaria exist either solitarily or in extensive colonies. Most cnidarians from the Hydrozoan class occur in the polypoid and medusoid stages during their life cycle. Most of the species from this class thrive in saltwater, but some of them live in freshwater bodies too. Members of this class go through both, the medusa as well as the polyp stage, through their life cycle. Freshwater Jelly (Craspedacusta sowerbyi), stinging hydroid (Macrorhynchia philippina), pink-hearted hydroids (Tubularia), Portuguese Man o' War (Physalia physalis), and freshwater polyps (Hydra) are a few examples of Cnidarian species that belong to this class. Cnidarians that belong to class Cubozoa are cube-shaped and have a distinct box-like appearance, hence, the name Cubozoa. More commonly known as Box jellyfish, these Cnidarian invertebrates boast of being the most poisonous and venomous creatures of the acquatic world. These are medusae Cnidarians. The nervous system of Box Jellyfish is the most developed amongst all Cnidarians.

► In layman terms, organisms that are radially symmetrical don't exhibit separate left or right sides. ► They do have distinguished dorsal and ventral (top and bottom) sides. When cut along any axis, these organisms result in the formation of identical segments. ► Radially symmetrical Cnidarians have their mouth, tentacles, and gastrovascular cavity aligned in a way that they appear the same when viewed from any angle or through any axis. ► Radial symmetry could be four-fold (tetramerism), as in the case of jellyfish, or it could be six-fold or eight-fold (hexamerism and octamerism), as in the case of corals and sea anemones, respectively. ► An interesting fact about Cnidarians is that they have just a single opening (cavity) in their body. They use the same opening for taking in food and excreting waste. ► This is a condition in which any Cnidarians' body and tentacles have two distinguished embryonic layers, namely, the ectoderm or epidermis (outer cellular layer), and the endoderm or gastroderm (inner cellular layer). ► The ectoderm is made up of epitheliomuscular cells (responsible for contraction and movement), cnidocytes (stinging cells), nerve cells (responsible for transmission and reception of sensory information), and interstitial cells (responsible for development of eggs and sperm). ► In most Cnidarians, the gastroderm is made up of glandular cells that secrete digestive enzymes. Some species might have a gastroderm which contains a limited number of cnidocytes, which are used in extreme cases where a struggling prey needs to be subdued. ► A non-cellular, translucent, jelly-like layer, the mesoglea separates the two epithelial cell layers. It acts as the internal skeleton which supports the Cnidarian species. It is elastic in nature, and needs the buoyancy of water to help in maintaining a stable form of the organism. Without this buoyancy, the mesoglea loses its stiffness. This causes the Cnidarians to collapse when taken out of the water. ► In some Cnidarian species like the hydra, the mesoglea is thinner as compared to that in jellyfish, in which the mesoglea amounts for the bulk of the organism. ► Since polyps are Cnidarians that are permanently attached to a substrate and cannot move, it is only the medusa-form Cnidarians that are capable of swimming and moving around freely. They do this by performing jet propulsion. ► The muscles present inside the umbrella-shaped bell squeeze the water out of the gastrovascular cavity, and at the same time the spring and buoyancy of the mesoglea triggers the recoil stroke. Repetitive contraction and recoil activity propels the organisms further in the desired direction. ► Swimming in water bodies where the current is too swift and strong becomes difficult for the medusae, since their body tissues are too thin and cannot stand the force of the current. ► Lesser-formed medusae and polyps like hydra and sea anemones are capable of limited movement by creeping, crawling, somersaulting, or wiggling their bases. ► None of the members of phylum Cnidaria possess respiratory organs like lungs or gills. Actually, Cnidarians do not need a respiratory system. ► This is because, every single cell of their body is always directly in contact with fresh-flowing water. So, at all times, fresh oxygen and stale carbon dioxide can easily be diffused into and from the organism's body. ► Similarly, Cnidarians do not need an excretory system either. Any kind of waste or stale material is eliminated from their body through the skin by diffusion. ► The respiration and exchange of gases in Cnidarians takes place over their entire body surface. The epithelial layers on the organisms absorb oxygen and release carbon-dioxide into the surrounding water. This exchange happens either through the gastrovascular cavity or by diffusion through the skin. Diffusion is the process in which there is movement of a substance from an area of high concentration of that substance to an area of lower concentration, until a state of equilibrium is achieved. ► All Cnidarians are carnivorous beings. ► They fulfill their dietary and nourishment needs in various ways, which include preying on crustaceans, fish, and sometimes even other Cnidarians. They filter food from the surrounding water, absorb dissolved organic chemicals from it, or symbiotically derive nutrients from the algae that grow within their cells. ► They get their prey by making use of cnydocytes to capture and poison their prey. The nematocysts present at the tips of the tentacles contain venom, which is injected into the body of the Cnidarians' victim after they sting it and paralyze it. The venom is fatal and results in instant death of the prey. Once the prey is killed, the Cnidarian swallows it through the gastrovascular cavity, where special cells within the gastroderm secrete chemicals that break down the food into smaller units. The nutrients are distributed over the entire Cnidarian's body by diffusion. ► Members of the phylum Cnidaria are capable of reproducing sexually as well as non-sexually. ► Asexual reproduction is a simple process. In some hydrozoan polyps, it is brought about by 'budding'. A bud falls off a parent organism and grows into a new polyp, whereas some Hydrozoan medusae can reproduce by splitting down the middle. Some Anthozoan Cnidarian species are even capable of reproducing asexually by splitting horizontally, resulting in new polyps, which then become capable of reproducing sexually as well. ► Medusae necessarily reproduce sexually. They produce sperms and eggs which are released into the surrounding water. Once the eggs are fertilized, they form new polyps, which grow further to produce new medusa, and the cycle continues.