Jellyfishes are animals that belong to Phylum Cnidaria, included in the class Scyphozoa. The name is also sometimes used for the related classes of medusae (Hydrozoa) and box jellyfish (Cubozoa). Almost all jellyfish live in the seas and though they lack true organ structures they feature specialized tissues.

The adult forms of these creatures are composed of 95-99% water. All species are found in each of the world’s oceans, with a few species living in fresh water. Most jellyfish are passive drifters that feed on small fish and zooplankton that become caught in their tentacles. Jellyfish have an incomplete digestive system, meaning that the same orifice is used for both food intake and waste expulsion. They are Coelenterates which means “hollow gut”, and is made up of a layer of epidermis, gastrodermis, and a thin jelly-like layer called mesoglea that separates the epidermis from the gastrodermis.

The jellyfish have two major body forms throughout their life. The first form is called the polyp stage and is characterized by a either a non-moving (sessile) stalk that catches food drifting by or a similar form that is free-floating. Their mouth and tentacles are located anteriorly, facing upwards. The second form looks like a saucer is called the medusa stage and is characterized by a round (radially symmetric) dome-shape body plan with food catching tentacles hanging down. It is this form which is most able to respond to and interact with its environment and is also the form most people are familiar with.

During the polyp stage, jellyfish do not have males or females, thus only asexual reproduction occurs. This happens in two ways: (1) budding, to produce other polyps; and (2) strobilating, to produce medusae. During budding, the egg or planula of the jellyfish attaches itself to a hard surface where it grows into its polyp form called scyphistoma. The scyphistoma then asexually produce many ephyra that look like round jagged disks and becomes a strobila. Then the ephyras detach themselves from the strobila and become mature free living medusae. At this stage is when they can reproduce sexually. The male will release their sperms into the water where the eggs will be fertilized.

Like all other cnidarians, jellyfish have stinging cells called cnidocytes which contains the stinging nematocysts on their tentacles. Whenever a prey comes in contact with a tentacle, hundreds to thousands of nematocysts fire one or another type of “hook and line” into the prey’s direction. These stinging cells are thus able to latch onto the prey and the tentacles bring the prey item into their large “mouth” for digestion.These cells are activated by simple but precise nervous system called nerve net which is located in the epidermis of the jellyfish. Impulses to these nerve cells are sent from the nerve rings that has collected information from the environment of the jellyfish through the rhopalial lappet, which is located around the animal’s body. Jellyfish also have eyes or ocellus that are cannot form images, but are sensitive to light. Jellyfish do not have a specialized digestive system, excretory system, respiratory system, and circulatory system. They are able to digest with the help of the gastrodermis that lines the gastrovascular cavity where nutrients from their food is absorbed. They do not need a respiratory system since their skin is thin enough that air can diffuse in and out of their body. They do not have a brain, a heart, a central nervous system, a skeletal system and also no bones and no blood. Jellyfish move using a hydrostatic skeleton that controls the water pouch in their body to manipulate their movements.

Most jellyfish are not dangerous to humans but a few are highly toxic, such as the Cyanea capillata. Contrary to popular belief the menacingly infamous Portuguese Man O’ War (Physalia) isn’t actually a jellyfish, but a colony of hydrozoan polyps.

Many aquaria, such as Monterey Bay Aquarium and Vancouver Aquarium, feature jellyfishes in display. Usually the tank’s background is blue with the animals illuminated by orange lights to produce a high contrast effect. These displays are often presented as a living art instead of showing the natural look of the animals which is usually very unattractive.