Jellyfish

Jellyfishes are animals that belong to Phylum Cnidaria, included in the class Scyphozoa. The name is also sometimes used for the medusae of Hydrozoa[?]. 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.

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. The second form is called the medusa stage and is characterized by a round (radially symmetric) 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. Jellyfish also reproduce sexually later in life after they differentiate into males and females. This occurs only when they have reached the medusa stage of their life cycle.

Like all other cnidarians, jellyfish have stinging cells called nematocysts on their tentacles. Whenever a prey item 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.

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.

Some Types of Jellyfish:

• Sea nettle[?]
• Portugese man-of-war
• Sea Wasp[?]

---

Jellyfish is also the name of an 80s / 90s pop band: see Jellyfish (band)[?]