Jellyfish Are Marine Invertebrates Term Paper

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Jellyfish are marine invertebrates which are part of the Scyphozoan class, and in turn the phylum Cnidaria; "members of this structurally simple marine group possess one of two body forms. Sea anemones, sea whips, corals and hydroids are polyps growing attached to rocks or other hard surfaces of the sea. Jellyfish and the Portuguese man-of- war are free-swimming medusa. Both body forms display radial symmetry with body parts radiating from a central axis. This symmetry allows jellyfish to respond to food or danger from any direction." Moreover, they have strange shapes and colors: they might look like a bell or an inverted bowl from 3 millimeters to 2 meters in diameter. Their bells contain small sense organs that respond to light and gravity.

Their jelly like bodies can be transparent or colored in pale blue, orange, brown, white or pink. In addition, other species may also be yellow, deep blue, bright purple, pale lilac, bright orange, deep red. Their bodies are made of 94-96% water, "a jelly producing substance enclosing its internal structure, from which the creature's tentacles are suspended. Each tentacle is covered with stinging cells (cnidocytes) that can sting or kill other animals: most jellyfish use them to secure prey or as a defense mechanism. The tentacles of some jellyfish can reach lengths greater than 100 feet. Regardless of their size or shape, most jellyfish are very fragile, often containing less than 5% solid organic matter."Buy full Download Microsoft Word File paper
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There are also other species, as Rhizostomae is, which bodies do not have tentacles at all. Because of the lack of basic sensory organs and of the brain, the jellyfish uses its nervous system and rhopalia more intensively, a thing that helps it identify and respond to different stimuli, such as light or odor, and orchestrate expedient responses. Their body comprises an outer layer, "epidermis," by which is covered, an inner layer "gastrodermis" which lines the gut, and the part between them is a layer of thick elastic jellylike substance named mesoglea -"middle jelly"-. The tiny beings' digestion is easily done: they have a simple digestive cavity called coelenteron which has the role of a gullet, stomach and intestine in the same time and is provided with one opening for the mouth and anus. The feeding process is completed with the help of four to eight oral arms situated near the mouth and used for transporting the food that has been caught with the help of the tentacles.

Sea Jellies have appeared millions of years ago on the planet, and they have survived a great period of time in their watery habitats. They live in all the oceans of the world, "from icy polar seas to tropical Pacific shores"; some of these species being even found in fresh waters, such as lakes and rivers. Moreover, they "are capable of withstanding a wide range of temperatures and salinities. Most live in shallow coastal waters, but a few inhabit depths of 12,000 feet."

Lifecycle and reproduction

In most of the cases, jellyfish pass through two distinct periods in their existence, each of them being identified with the help of their different body form; thus the first one is named the polyp stage and it is distinct through the fact that "the jellyfish takes the form of either a sessile stalk which catches passing food, or a similar free-floating configuration. The polyp's mouth and tentacles are located anteriorly, facing upwards." In the second stage, the jellyfish receives the name of medusa. Its body will catch a symmetric, umbrella-shaped body, named by the specialists "bell," its tentacles being hanged from the border of this so-called bell.

Like many other invertebrates, jellyfish are dioeciously, meaning that they are either male or female. The reproduction method is simple and applicable to almost all species: the male releases his sperm into the surrounding water and the female's fertilization is indirectly made, when she swallows the sperm, allowing a new life cycle to begin. Even though, other species, such as moon jellies, reproduce themselves with the help of their eggs, which are kept, in order to be protected, in pits and on the oral arms.

The result of the reproduction is the planula, which has a larval form, is small and covered with cilia. The next stage in its life consists in developing into a polyp. At this stage the body would moment in which it begins to reproduce itself in an asexual manner, by budding, thus receiving the name of segmenting polyp, or a scyphistoma. In this way new scyphistomae might be produced by budding or new-born jellies called ephyra may be formed. More interesting, "many jellyfish can bud off new medusae directly from the medusan stage."

Most jellyfish's life cycle lasts around six months, 2 and a half months being the most oftenly met period of time.

Defense and feeding mechanisms

Jellyfish protect themselves with the help of their tentacles or oral arms, on which there can be found microscopic nematocysts; usually, these "defense tools" are each provided with a "cnidocil" containing a capsule with a filament and barbs on the exterior. When it comes into contact with a strange body, this filament will inject it with toxins. Its role not being only a defense system, this mechanism is also used in the feeding process, as the jellyfish deadly injects its "victim" with its toxic substance, sometimes even pulling it directly into its mouth.

A tentacle is most of the times provided with have hundreds or thousands of nematocysts found at the level of the epidermis. As it has been outlined above, triggers of nematocysts are activated when the animals touch another object. The pressure exercised inside the nematocyst forces the stinging thread to be released. Nematocysts have the role of small harpoons, firing into prey, injecting paralyzing toxins. Even though, only small beings are paralyzed or killed by jellyfish, but some of them might be even dangerous for humans.

Body systems

The jellyfish's nervous system is located at the level of its epidermis and it is called "nerve net." One of its main functions is that of detecting the touch of other animal beings. The functioning mechanism of this apparatus is quite simple: nerve rings collect data from the environment of the invertebrate through the rhopalial lappet, and then send it to the nerve cells through body impulses. Even though they cannot perceive images, Jellyfish have ocelli that are able to detect light; practically, "the jellyfish bases its judgment on sunlight shining on the surface of the water."

As it has been outlined above as well, jellyfish do not have digestive, central nervous, respiratory or circulatory systems; their digestion is produced instead through gastrodermis, something such as a protection vale for the cardiovascular cavity, where food is being processed. "Jellyfish are carnivorous, feeding mostly on a variety of zooplankton, comb jellies and occasionally other jellyfish. Bigger species, instead, are capable of capturing and devouring large crustaceans and other marine organisms." Even though, their most common food is made of small animals, like shrimp or plankton. The animals they usually eat are microscopic, sometimes unable to be seen by the human eye. The feeding process is done with the help of nematocysts, small organs present on the tentacles and oral arms I have mentioned earlier, and also through their poisoning system. They are not provided with the ability to hunt, so they prefer to wait for small animals to come into contact with their tentacles. On the other hand, they are not able to carry heavy food quantities in their bodies, so the digestion process has to be made fast, leaving aside the waste matter, because otherwise they would not be able to float anymore.

Moreover, the tiny beings breathe with the help of their skin, this being the level at which the oxygen is absorbed into their bodies. Jellyfish move into water by floating, but it can use its muscles to control the water and in this way to move vertically, through rhythmic pulsations. In what their horizontal movements are concerned, the jellyfish are totally dependent on oceans' water and wind.

The tiny beings are covered by ectoplasm, a kind of plasma from a jelly like material, which has in its composition protein granules and other types of similar organic substances.

Blooms and grouping

Jellyfish, usually a number of hundreds or even thousands of animals, group into "smack"; in addition, they are able to group themselves into "swarms" or "blooms." The formation of these groups is quite complicated, since it depends on "ocean currents, nutrients, temperature and oxygen content. Jellyfish will sometimes mass breed during blooms. Jellyfish population is reportedly raising major ecological concerns for a possible jellyfish outbreak."

It has many times been contended that "these blooms may be attributed to mankind's impact on marine life," which has lead to the decreasing of the number of fish in the marine waters, and jellyfish have taken themselves the role to replace them.

On the other hand, these groups might be seen as a follow-up of the waters'… [END OF PREVIEW] . . . READ MORE

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