Louis J Sheehan 654365

Jellyfish are marine invertebrates belonging to the class Scyphozoa of the phylum Cnidaria. They can be found in every ocean in the world and in some fresh waters. The use of the term "jellyfish" is actually a misnomer since scyphozoans are not fish, which are vertebrates. Although incorrect, the term is also commonly-applied to some close relatives of true scyphozoans, such as the Hydrozoa and the Cubozoa.

The body of an adult jellyfish consists of a bell shape producing jelly and enclosing its internal structure, from which tentacles are suspended. Each tentacle is covered with cells called cnidocytes, that can sting or kill other animals. Most jellyfish use these cells to secure prey or for defense. Others, such as the Rhizostomae, do not have tentacles at all.

Jellyfish lack basic sensory organs and a brain, but their nervous systems and rhopalia allow them to perceive stimuli, such as light and odor, and respond quickly. They feed on small fish and zooplankton that become caught in their tentacles. Most jellyfish are passive drifters and slow swimmers, as their shape is not hydrodynamic. Instead, they move so as to create a current forcing the prey within reach of their tentacles. They do this by rhythmically opening and closing their bell-like body. Their digestive system is incomplete: the same orifice is used to take in food and expel waste. The body of an adult is made up of 94–98% water. The bell consists of a layer of epidermis, gastrodermis, and a thick, intervening layer called mesoglea that produces most of the jelly.

Most jellyfish have tendrils or oral arms coated with thousands of microscopic nematocysts. Generally, each nematocyst has a "trigger" (cnidocil) paired with a capsule containing a coiled stinging filament armed with exterior barbs. Upon contact, the filament rapidly unwinds, launches into the target, and injects toxins. The animal can then pull its prey into its mouth, if appropriate.

Although most jellyfish are not perniciously dangerous to humans, a few are highly toxic, such as Cyanea capillata. Contrary to popular belief, the menacingly infamous Portuguese Man o' War (Physalia) is not a jellyfish but a colony of hydrozoans. Similarly, the box jellies, notorious along the coast of Australia, are cubozoans, not true scyphozoan jellyfish. Irrespective of the sting's toxicity, many people stung by them find them very painful and some people may suffer anaphylaxis or other severe allergic reactions, similar to allergies to bee stings.

A jellyfish detects the touch of other animals using a nervous system called a "nerve net", located in its epidermis. Touch stimuli are conducted by nerve rings, through the rhopalial lappet, located around the animal's body, to the nerve cells. Jellyfish also have ocelli: light-sensitive organs that do not form images but are used to determine up from down, responding to sunlight shining on the water's surface.

Jellyfish do not have a specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. They digest using the gastrodermal lining of the gastrovascular cavity, where nutrients are absorbed. They do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement and mostly free-float, but can use the hydrostatic skeleton of the water pouch to accomplish vertical movement through pulsations of the disc-like body.

The outer side of a jellyfish is lined with a jelly-like material called ectoplasm (ecto meaning outer and plasma meaning cytoplasm). The ectoplasm typically contains a smaller amount of protein granules and other organic compounds than inner cytoplasm, also referred to as endoplasm (endo meaning inner).
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