|Etymology:- From the Greek knide for nettle.|
Characteristics of Cnidaria:-
2)Body multicellular, few tissues, some organelles.
3)Body contains an internal cavity and a mouth.
4)Two different forms exist, medusa and polyp
5)Reproduction is asexual or sexual.
6)Has a simple net like nervous system.
7)Has a distinct larval stage which is planktonic.
8)Lives in aquatic environments, mostly marine.
9)Mostly carnivorous otherwise filter feeders.
10)May have a minimal skeleton of chiton or calcium carbonate.
The Cnidaria (pronounced nidaria)as a group of animals are well known to many people under their common names, Sea Anemones, Corals and Jellyfish are all Cnidarians as are Hydras, Sea Whips, Sea Fans and Sea Pansies. They are linked together by their carnivorous feeding habits their simple anatomical design and the possession of nematocysts, though one species of Ctenophora possesses nematocysts as well.
The name Cnidaria has now pretty much replaced the older term of Coelenterata (pronounced selenterata) which these days is often applied to both the Cnidaria and the Ctenophora together, these two phyla are also known as the Radiate Animals because they both have radial or biradial symmetry. The word Cnidaria refers to Cnidocysts, specialised cells which contain the Nematocysts, the stinging organelles that allow the Cnidaria to subdue their prey.
The Cnidaria are the oldest of the true metazoan phyla. A fossil Hydrozoan from South Australia called Ediacara is 700 million years old, while numerous fossil Cnidarians exist from the Cambrian 500 million years ago. The Cnidarians, particularly the corals often make up an important component of the shallow marine fauna of tropical and subtropical seas. All the Cnidaria are aquatic and nearly all are marine. Corals because of their shallow marine environment and their habit of accumulating a mineralised skeleton (coralite) tend to fossilize well and we know quite a bit about their evolution.
The Cnidarian body is basically a U shape with intact walls that surround a central digestive area and a mouth at the opening, generally surrounded by tentacles, there is no distinct anus. In Anemones the mouth faces up, and in Jellyfish it faces down. The Cnidarians show a more complicated arrangement of cell layers as well as a greater range of cell types than the Porifera. Their bodies show two distinct layers of cells and thus they are called 'Diploblastic animals'. The two cell layers are an outer Epidermis or Ectoderm, and an inner Gastrodermis or Endoderm. These two layers are separated by the mesoglea a non-cellular fibrous jelly like material that is thin in some groups such as the Hydras but can be quite thick in other such as the Jellyfish where it helps provide negative buoyancy (makes the animal more likely to float). The ectodermis consists of five basic cell types, Epitheliomuscular cells which supply some of the muscular capabilities of the animal, Interstitial cells which are basic cells that give rise to the other cell types, Cnidocysts (see below), Mucous glands and sensory or nerve cells. The endodermis consists of three or four basic cell types Gastromuscular cells which help digest food items and provide some muscle power, Gland cells that secrete enzymes for digestion, Mucous cells and in Anemones but not in Hydras, Cnidocytes.
One of the most important distinguishing characteristics of the phylum are the Nematocysts. Nematocysts, and their enclosing Cnidocysts come in about 24 different forms, the differences play a functional role in the classification of the phylum. A Cnidocyst is a cell that secretes a nematocyst within it. A basic Nematocyst is a capsule made of something like chitin within which rest a coiled thread. This thread can be shot out of the capsule to encounter prey items, or in some cases to repel predators. The Cnidocyst has either a modified flagellum called a Cnidocil, or a cone as a sensory trigger. If this trigger is touched the nematocyst thread is rapidly ejected. Nematocyst threads come in 3 basic types. The fundamental nematocyst is a thin tubular thread with barbs at the far end, though there may be barbs near the base as well. When the nematocyst is discharged, the barbs penetrate the skin of the prey and a toxin can be injected. Ptychocysts are uncommon, occurring only in the Ceriantharians, they lack spines or barbs but are adhesive and can be used to line the tubes the Ceriantharians live in as well as to entangle prey. Spirocysts also lack barbs or spines, they are an enclosed tube that is adhesive, they are used to trap prey in a tangled net of sticky threads.
Though a few people get stung by jellyfish every year, and a very few of these stingings may be fatal, the only Cnidarians to have a real impact on human beings are the corals. Corals and the reefs they form are important breeding areas for fish, some of which are commercially important, they are also important in terms of biodiversity because of the wide range of creatures that live preferentially, or only in or near coral reefs. Thirdly they are of importance because of their beauty and thus their use as a tourist attraction, coral reefs are among the most beautiful and colourful places on the planet. It is a sad truth, and a condemnation of humanity that these structures which take hundreds or thousands of years to develop are rapidly being destroyed in one way or another by mankind, over 75% percent of reefs are now suffering some sort of damage, many are completely dead. To add to their troubles reef building corals, which live in a delicate balance with the dinoflagellate Symbiodinium microadriaticum have a very narrow temperature range within which they produce new coral skeleton faster than the rest of nature whereas it away. This temperature range is 23 to 29 degrees C, or 73 to 84 degrees F. There are fears that global warming could have a disastrously destructive effect on the world's remaining coral reefs.
The Cnidaria come in two basic forms, a 'Polyp' form typified by the Sea Anemones and a 'Medusa' form typified by Jellyfish. Generally speaking Polyps are tube shaped and sedentary with a ring of tentacles around the mouth, Medusae are umbrella or bell shaped, free living and have a central projection on the inside of the umbrella which supports the mouth and their tentacles around the rim of the umbrella.
The Cnidarians are either carnivores or omnivorous filter feeders.The the carnivorous forms do not hunt their prey, instead they use various 'sit and trap' or 'float/swim and trap' strategies, using their Nematocysts, which are not only found on the stinging tentacles but can be all over the animals body, to stun and or kill their prey. There are about 10, 000 species of Cnidarians divided between 3 classes Hydrozoa, Scyphozoa and Anthozoa.
|Hydrozoa||Solitary or colonial, sessile as adult||Sexual polyps and asexual medusa either of which may be absent.||Freshwater and Marine||Hydra, Obelia, Physalia, Tubularia|
|Scyphozoa||Solitary, nearly all free swimming||Sexual medusa with a reduced or absent polyp||Marine only||Aurelia, Cassiopeia, Chironex, Rhizostoma|
|Anthozoa||Solitary or colonial sessile as adult||Polyp only||Marine only||Adamsia, Cerianthus, Gorgonia, Renilla|