CNIDARIA (Coelenterates): The name Cnidaria comes from the Greek word "cnidos", which means stinging nettle.
Cnidocytes containing nematocysts (cnidae) are characteristic of (and limited to) this phylum. This characteristic is a true morphological synapomorphy.
In other words, if an organism can manufacture its own nematocysts, by definition it is a cnidarian.
Example: Myxozoans are species that parasitizes freshwater fish especially trout. It digests the fish cartilage leaving the fish deformed. Inflammation develops from the infection, puts pressure on the nerves and disrupts balance causing the fish to swim in circles, a disease known as whirling disease. Spores released from the decaying carcass may survive for up to 30 years in the mud. Spores are eaten by Tubifex worms which are eaten by fish.
http://www.sci-news.com/genetics/myxozoans-parasites-micro-jellyfish-cnidarians-03443.html
Simply scan this article for great pictures of these critters, examine the life cycle to see how morphology really changes during the life cycle. https://en.wikipedia.org/wiki/Myxobolus_cerebralis
Characteristics
So what characteristics do most (and more typical) cnidarians have in common, and which of these do they share with other animals?
The symmetry is primarily radial symmetry, although in some it can be modified as biradial or some other form.
Cnidarians are described as diploblastic (2 layers) and are characterized by the tissue level of organization.
Cnidarians are essentially made of two cell layers. The outer ectoderm, or epidermis, contains the cnidocysts, the stinging cells that are characteristic of the phylum. The inner endoderm, or gastrodermis, lines the gut, which in some cnidarians may be divided up by septa (as in the Anthozoa) or elaborated into branching canals (as in many Scyphozoa). This layer allows extracellular digestion. These groups, probably because of their larger size (than Hydrozoa, the third group we will discuss) have become more "impressive" feeders.
In between epidermis and gastrodermis is the mesoglea or mesenchyme, a layer of jellylike substance which contains scattered cells and collagen fibers. Mesoglea is variable from very thick in anemones and most jellyfishes, to quite thin in some of the hydroids. The mouth is often, but not always, surrounded by a ring of tentacles.
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An aside on terminology (There is a lot of controversy among the terms mesohyll, mesoglea or mesenchyme. Originally mesenchyme (middle juices) was used to denote a "middle" layer in Porifera or Cnidaria that was primarily ectodermally derived that had some cells in it; mesoglea refers to the jelly like matrix in which the cells where found. Soon researchers were using mesoglea for the middle layer in forms that had mostly matrix, mesenchyme for the forms that had many cells in their matrix. A second meaning was added by embryologists working on vertebrate and other triploblastic groups, causing some anatomists to become more cautious and start coining terms such as mesohyll for sponges. We will continue to use the terms interchangeable and so defer in some sense to their original meanings.)
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Polymorphism in life cycle is common in most species (think adult frog versus tadpole). Both polyp and medusa stage are found in many species, but this varies from one group to another. In some groups, only the polyp occurs and in other groups the medusa dominates.
The various groups (clades or are distinguished by life cycle differences more than any other varying characteristic
The different types of cnidarians
CLADE or Class Scyphozoa: "True" Jellyfish.
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An aside on terminology
(Jellyfish is a sea animal with a soft oval almost transparent body, and so this term includes other creatures including some snails sometimes. It certainly includes all the cnidarian medusae and some floating polyp state of the Hydrozoa. Taxonomically the term is most used in conjunction with the large medusae of this clade and the Cubozoa which are often considered part of this clade.)
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Medusa is dominant, polyp is reduced.
The sexes are often separate (dioecious). Sperm are released from mouth of male into the water and are filtered from the water by the female and fertilization is internal. Embryos develop into planula larvae in specialized brood pouches in the folds of the mouth lobes. Planulas are ultimately released and attach to overhanging rocks where they develop into polyps (scyphistoma). Scyphistomas bud-off additional polyps which ultimately undergo strobilation, a progressive constriction into a stack of saucer-like medusae (ephyrae) which subsequently separate.
It is important to recognize that Scyphyzoa have a polyp stage, although it is small and short lived in most cases. http://vimeo.com/40232821
Other characteristics:
These are among the largest of the cnidarian individuals known. In polar waters some species can reach bell diameters of 8 ft. and have tentacles 130 ft. long.
Example: Aurelia
Aurelia is a representative form. It may reach a diameter of 1 meter. Aurelia has 4 trailing mouth lobes (oral arms) which each contain a ciliated groove and many nematocysts. The scyphozoan gastric cavity is divided into four gastric pouches that contain a tentacle-like projection of gastrodermis called a gastric filament which contain numerous gland cells that secrete digestive enzymes. The gastric pouches contain nematocysts. Again these are impressive feeders.
There are two (or more) nerve nets with ganglia, one which serves the whole surface of the animal and one that serves only the "bell." The latter conducts impulses more rapidly and is concerned with swimming. They also have rhopalium, a complex of sensory structures (one statocyst, two simple eyes, and two chemosensory pits).
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The Cubozoa are a class which are generally small and transparent. They are sometimes described as the Cubomedusae or an group of scyphozoans (depends on book) . The bell is almost perfectly square in cross section. They have the most complex and sensitive rhopalial eyes of any Cnidarian (with a lens, retina, and thousands of sensory cells) and are strongly positively phototactic. Common names are "box jellies" and "sea wasps", named respectively for their square cross section and their powerful sting.
Visit this website to learn more about the cubozoa_http://www.ucmp.berkeley.edu/cnidaria/cubozoalh.html__ _________________________________________________________________________________________
CLADE or Class Anthozoa, Anemones and corals. Polyp only. (about 6,500 spp.) These are all marine. There are no fresh water anthozoans.
Examples: The CLADE or Order Actiniaria are the "sea anemones". All are large solitary polyps with the typical polyp organization of column, oral disk, mouth, tentacles, basal disk. While most anemones are sessile, a few can burrow into soft substrates and a few are pelagic, using a floating device secreted by the basal disk.
The tentacles occupy the oral disc around the mouth. Mouth communicates to gastric cavity through a pharynx. Although external symmetry is radial, the internal symmetry is biradial or bilateral. The mesoglea is thick and a true cellular layer. The gametogenic tissue is embedded within the mesenteries between the musculature and the filaments.
The gastrovascular cavity is divided by numerous septa (called mesenteries) which increase the surface area for absorption of the products of digestion.
The sphincter muscle around the margin of the oral disk is used to contract the oral disk. The anemone affixes itself to the substrate using its pedal disc. They have both longitudinal and circular "muscles" that can be used to bend the animal. In an emergency, all the longitudinal muscles can be contracted while the mouth is open. The animal becomes very flat and almost unrecognizable as a anemone.
There can be fierce competition for space among individuals. Most anemones have nematocyst-studded acontia that protrude from the interior edges of these mesenteries.
Some species fight with specialized tentacles or bulges at the pedal base that contain highly poisonous nematocysts. http://vimeo.com/40198090
Eggs and sperm usually ejected through mouth and fertilization is generally external, although some species fertilize internally and brood their young either internally, or attached to the outer surface.
This group contains the corals, which contains some very important reef building species.
Corals contain symbionts that produce photosynthetic products and aid in depositing calcium carbonate. The specific details of this cooperation are unknown. We can demonstrate that the rate of calcium carbonate deposition is greater in light than darkness. Symbionts gain access to CO2 and other nitrogenous products of digestion produced by anthozoans and protection
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Major stresses to coral reefs are: natural forces that they have coped with for millions of years; direct human pressures, including sediment and nutrient pollution from the land, over-exploitation and damaging fishing practices, engineering modification of shorelines; and the global threats of climate change causing coral bleaching (map below show hotspots), rising sea levels are potentially threatening the ability of corals to form skeletons in more acid waters. Read more about Coral bleaching.
Hydrozoa
Probably the cnidarian most encounter before college is Hydra, more specifically Hydra littoralis. It is named after the monster in Greek legend with many heads who grew two more whenever one was cut off. It was finally slain by Hercules who cauterized each neck after severing its head. The regenerative powers of Hydra are quite remarkable, a single individual may be cut into many pieces and provided that each piece contains a portion of the two body layers, ectoderm and endoderm, it will develop into a complete animal. However, this specis is an unusual hydrozoan because it only has a polyp stage. There is no medusa stage. Testes and ovaries simply develop at certain times on the same or different polyp depending on the Hydra species of interest.
Craspedacusta sowerbyi is our only common freshwater jellyfish. This species is more typical in that it has a polyp and a medusa, although here is it the polyp stage that is reduced in size. There are however many different types of polyps that can develop in response to environmental conditions. As expected the medusa reproduces sexually.
Obelia is an example of a typical colonial hydrozoan and also shows a very typical life cycle with polyp dominating but medusa present. It also sports a perisarc, a transparent protective tube, found in most species, composed of polysaccharides, protein and chitin. If it covers the polyps, it is known as a theca.
The following video shows hydrozoan medusae being released. http://vimeo.com/12949246 Note how much smaller the hydrozoan medusae are than those produced by Scyphozoans.
Also most hydrozoans are colonial and show polyp (also known as zooid) polymorphisms. Most at least have gastrozooids (feeding individuals) and gonozooids (reproductive individuals) as shown in the diagram of an Obelia colony above.
Physalia is commonly known as the Portuguese Man of War, and in this species and other siphonophores, the colony can be mistaken for one individual organism. Most species of hydrozoans are small. However colonial species can consists of hundred of individuals and so cover several meters of surface. The free floating or swimming colonies of siphonophores however are among the largest of hydrozoans, with the size of individual colonies rivaling that of a large scyphozoan medusa.
Learn more about siphonophores at http://www.siphonophores.org/index.php Explore colonial types, and body plans,
Phylogeny
The body plan is very simple for many forms and symmetry is radial. These are two reasons why the group is not considered a suitable candidate for giving rise direct ancestor to biradial animals. Cnidaria are considered a successful side branch. The anthozoans are considered the most primitive and the hydrozoans most advanced. This is based on a cladistic analysis (Schuchert, 1993) and available molecular data. It would make the medusa the more derived stage. This seems consistent with the fossil record of the Cnidaria, which but further studies will be needed to refine this picture of cnidarian evolution.
