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Facts About Jellyfish

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Jellyfish - Scyphozoa

Jellyfish - Scyphozoa

Photo © Sieber / Dreamstime.

In this article, you'll learn interesting facts about jellyfish and find out about their unique characteristics, their life cycle and their evolutionary history.

Jellyfish get a lot of negative press for the threats they can pose to swimmers and holiday makers visiting beaches around the world. But there's more to jellyfish than a nasty sting. In this article we'll take a closer look at these unique creatures.

FACT: Jellyfishes are made up of more than 95% water.

Their bodies are soft and lack a skeletal structure or outer shell. They are delicate and easily damanged. Jellyfishes require water to help support their body and if removed from their aquatic surroundings, they collapse and die.

FACT: Jellyfish are radially symmetrical.

Jellyfish are symmetrical about a central axis that runs through the length of their body, from the top of their bell to the ends of their tentacles. They have a top and a bottom but they lack a left and right side and as a result differ from many other types of animals (such as mammals, reptiles, fish, birds, and arthropods) which exhibit bilateral symmetry.

FACT: A jellyfish has a simple digestives system with only one opening.

A jellyfish takes food in through its mouth which is located on the underside if its bell. Food is digested in a sac-like structure called a coelenteron or gastrovascular cavity. Waste material is passed out through the mouth.

FACT: A common analogy used to describe the delicate way jellyfish pounce through the water likens the jellys' movements to 'a simple form of jet propulsion'.

To move forward, jellyfishes take water into their muscular bell and then squirt it out behind them, creating a jet of water that propels the jelly forward. In addition to this form of movement, jellies also drift on water currents to move.

FACT: Jellyfishes have no brain, no blood, and no nervous system.

Their senses are primitive and consist of a neural net, eye spots that can sense light from dark, and chemosensory pits that help them identify potential prey.

FACT: A jellyfishes' body consists of three layers.

The outer layer is called the epidermis, the inner layer which lines the gastrovascular cavity is called the gastrodermis, and the middle layer consists of a thick substance called the mesoglea.

FACT: Thousands of nematocytes are located on the tentacles, feeding arms, and mouth of a jellyfish.

Nenatocysts consist of a capsule that holds a hollow barbed coil, a vemon sac, and chemo-sensitive trigger hairs that detect when something edible brushes against them. When potential prey brushes against the trigger hairs, the nematocytes expel the coiled barb and inject venom into the victim through the hollow thread. The venom immobilizes the prey and the jellyfish uses its oral arms to move the prey into its mouth where it is passed through to the coelenteron for digestion.

FACT: Jellyfish belong to the Phylum Cnidaria.

This group of animals, all radially symmetrical, includes corals, sea anemones, hydras, and jellyfish.

FACT: There are about 200 species of True Jellyfishes.

True Jellyfish are species belonging to the Class Scyphozoa. Examples of True Jellyfish include Moon Jellies, Mediterranean Jellyfish, Sea Nettles, Lion's Mane Jellyfish, Blue Jellies, and many other lesser known species. The Class Cubozoa includes about 20 species not considered to be True Jellyfish. The Class Cubozoa is also referred to as box jellyfish. The most imfamous of the Cubozoa is the Sea Wasp, a creature with a deadly sting that inhabits the waters off the coast of Australia.

FACT: The species Craspedacusta sowerbii is sometimes referred to as the only species of freshwater jellyfish, although it is not a true jellyfish.

Craspedacusta sowerbii belongs to the Class Hydrozoa (the group of animals that includes the hydra), not the Class Scyphozoa.

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