Cnidarians are invertebrates such as jellyfish and corals. They belong to the phylum Cnidaria. All cnidarians are aquatic. Most of them live in the ocean. Cnidarians are a little more complex than sponges. They have radial symmetry and tissues. There are more than 10, cnidarianspecies. They are very diverse, as shown in Figure below. All cnidarians have something in common. A nematocyst is a long, thin, coiled stinger. It has a barb that may inject poison. These tiny poison "darts" are propelled out of special cells.
They are used to attack prey or defend against predators. Cnidarian Nematocyst. A cnidarian nematocyst is like a poison dart. It is ejected from a specialized cell. There are two basic body plans in cnidarians.
They are called the polyp and medusa. Both are shown in Figure below. The polyp has a tubular body and is usually sessile. The medusa plural, medusae has a bell-shaped body and is typically motile.
Some cnidarian species alternate between polyp and medusa forms. Other species exist in just one form or the other. Cnidarian Body Plans. Cnidarians may exist in the polyp left or medusa right form.
The body of a cnidarian consists of two cell layers, ectoderm and endoderm. The cellssurround a digestive cavity called the coelenteron see Figure below. Cnidarians have a simple digestive system. The single opening is surrounded by tentacles, which are used to capture prey. The tentacles are covered with nematocyst cells.
Digestion takes place in the coelenteron. Nutrients are absorbed and gases exchanged through the cells lining this cavity. This cavity is divided into several chambers by longitudinal septa called mesenteries. Each mesentery consists of a fold of gastrodermal tissue with a layer of mesoglea between the sheets of gastrodermis.
Mesenteries do not divide the gastrovascular cavity completely, and the smaller cavities coalesce at the pharyngeal opening. The adaptive benefit of the mesenteries appears to be an increase in surface area for absorption of nutrients and gas exchange, as well as additional mechanical support for the body of the anemone.
Sea anemones feed on small fish and shrimp, usually by immobilizing their prey with nematocysts. Some sea anemones establish a mutualistic relationship with hermit crabs when the crab seizes and attaches them to their shell. In this relationship, the anemone gets food particles from prey caught by the crab, and the crab is protected from the predators by the stinging cells of the anemone. Some species of anemone fish, or clownfish, are also able to live with sea anemones because they build up an acquired immunity to the toxins contained within the nematocysts and also secrete a protective mucus that prevents them from being stung.
The structure of coral polyps is similar to that of anemones, although the individual polyps are usually smaller and part of a colony, some of which are massive and the size of small buildings. Coral polyps feed on smaller planktonic organisms, including algae, bacteria, and invertebrate larvae. Some anthozoans have symbiotic associations with dinoflagellate algae called zooxanthellae. The mutually beneficial relationship between zooxanthellae and modern corals—which provides the algae with shelter—gives coral reefs their colors and supplies both organisms with nutrients.
This complex mutualistic association began more than million years ago, according to a new study by an international team of scientists. That this symbiotic relationship arose during a time of massive worldwide coral-reef expansion suggests that the interconnection of algae and coral is crucial for the health of coral reefs, which provide habitat for roughly one-fourth of all marine life.
Reefs are threatened by a trend in ocean warming that has caused corals to expel their zooxanthellae algae and turn white, a process called coral bleaching. Male or female gametes produced by a polyp fuse to give rise to a free-swimming planula larva. The larva settles on a suitable substratum and develops into a sessile polyp. The medusa is the prominent stage in the life cycle, although there is a polyp stage in the life cycle of most species.
Most jellies range from 2 to 40 cm in length but the largest scyphozoan species, Cyanea capillata , can reach a size of two meters in diameter.
Scyphozoans display a characteristic bell-like morphology Figure. In the sea jelly, a mouth opening is present on the underside of the animal, surrounded by hollow tentacles bearing nematocysts. Scyphozoans live most of their life cycle as free-swimming, solitary carnivores.
The mouth leads to the gastrovascular cavity, which may be sectioned into four interconnected sacs, called diverticuli. In some species, the digestive system may branch further into radial canals.
Like the septa in anthozoans, the branched gastrovascular cells serve two functions: to increase the surface area for nutrient absorption and diffusion, and to support the body of the animal. In scyphozoans, nerve cells are organized in a nerve net that extends over the entire body, with a nerve ring around the edge of the bell. Clusters of sensory organs called rhopalia may be present in pockets in the edge of the bell.
Jellies have a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water, as well as to draw in food from the water as they swim. Scyphozoans have separate sexes. The gonads are formed from the gastrodermis and gametes are expelled through the mouth. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form. These polyps may bud to form additional polyps or begin immediately to produce medusa buds.
In a few species, the planula larva may develop directly into the medusa. The life cycle Figure of most scyphozoans includes both sexual medusoid and asexual polypoid body forms. However, cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans. A prominent difference between the two classes is the arrangement of tentacles.
The cubozoans contain muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. In some cases, the digestive system may extend into the pedalia. Nematocysts may be arranged in a spiral configuration along the tentacles; this arrangement helps to effectively subdue and capture prey. Cubozoans include the most venomous of all the cnidarians Figure. These animals are unusual in having image-forming eyes, including a cornea, lens, and retina.
Because these structures are made from a number of interactive tissues, they can be called true organs. Eyes are located in four clusters between each pair of pedalia. Each cluster consists of four simple eye spots plus two image-forming eyes oriented in different directions.
How images formed by these very complex eyes are processed remains a mystery, since cubozoans have extensive nerve nets but no distinct brain. Nontheless, the presence of eyes helps the cubozoans to be active and effective hunters of small marine animals like worms, arthropods, and fish.
Cubozoans have separate sexes and fertilization occurs inside the female. Planula larvae may develop inside the female or be released, depending on species. Each planula develops into a polyp. These polyps may bud to form more polyps to create a colony; each polyp then transforms into a single medusa.
Class Hydrozoa Hydrozoa is a diverse group that includes nearly 3, species; most are marine, although some freshwater species are known Figure. Most species exhibit both polypoid and medusoid forms in their lifecycles, although the familiar Hydra has only the polyp form. The medusoid form has a muscular veil or velum below the margin of the bell and for this reason is called a hydromedusa. In contrast, the medusoid form of Scyphozoa lacks a velum and is termed a scyphomedusa. The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined by the gastrodermis.
This penetrates the prey's outer covering and injects it with venom. The prey is then moved to the mouth by a tentacle.
Their prey can range in size from plankton to animals several times larger than themselves. Some obtain their energy from algae that that live in their bodies and a few are parasites. Other Cnidaria, including the corals, get their nutrients from symbiotic algae within their cells.
Predators of Cnidaria include sea slugs, sea stars for example, the Crown of Thorn which can devastate coral reefs , nudibranchs, fish including butterfly fish and parrot fish, which eat corals and marine turtles and sunfish, which eat jellyfish. Plastic bags floating in the oceans are dangerous to turtles which often can mistake them for jellyfish and their guts can become blocked by them.
For example coral, inside the sac of each coral polyp lives a singlee-celled algae called zooxanthellae. The algae produces oxygen and energy sugars that the coral polyp needs to live and, in return, the polyp produces carbon dioxide and other substances the algae needs. That is why coral reefs grow so near the surface of the water where the algae can get sunshine for photosynthesis.
Many Cnidaria live in colonies made up of large numbers of individuals joined together in some way. These individuals called zooids can either be directly connected by tissues or share a common exoskeleton made from chiton or calcium carbonate. In the Cnidaria sexual reproduction often involves a complex life cycle with both polyp and medusa stages. For example in Scyphozoa jellyfish and Cubozoa box jellies a larva swims until it finds a good site, and then becomes a polyp. This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called strobilation.
The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically.
The adults have gonads in the gastroderm, and these release ova and sperm into the water in the breeding season.
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