Facts About Jellyfishes


Jellyfish belong to the Phylum Cnidaria. This group of animals, all radially symmetrical, includes corals, sea anemones, hydras, and jellyfish. Jellyfish are literally boneless, brainless, and heartless, and most are transparent. Though they might not have brains, jellyfish do have a nervous system, or, nerve net, with receptors that can detect light, vibrations, and chemicals in the water.

Some jellyfish have ocelli, which are eye-like organs that are light-sensitive and can detect up and down motions. Ocelli appear as dark pigmented spots on the jellyfish. Jellyfish can reproduce both sexually and asexually.

There are about 200 species of True Jellyfishes. True Jellyfish are species belonging to the 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 Cubozoa include about 20 species not considered to be true jellyfish. The Cubozoa is also referred to as box jellyfish.

The box jellyfish has more advanced vision: its 24 eyes give it a 360-degree view of its environment. It is also the world's most dangerous jellyfish, and the most venomous marine creature. Certain species of box jellyfish can kill a person in just a couple of minutes.

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 damaged. Jellyfishes require water to help support their body and if removed from their aquatic surroundings, they collapse and die.

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.


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.

A common analogy used to describe the delicate way jellyfish pounce through the water likens their 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. Despite their poisonous defenses, jellyfish have many predators. Sharks, tuna, swordfish, sea turtles, and even salmon have been known to prey upon the jellyfish.

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.

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.


Thousands of nematocytes are located on the tentacles, feeding arms, and mouth of a jellyfishNenatocysts consist of a capsule that holds a hollow barbed coil, a venom 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.

More than any other creature, jellyfish rule the water. The scyphozoan class of jellyfish are found in every ocean in the world, and the hydrozoan class can flourish in freshwater lakes and ponds.


>>Watch the following video for more info;

Posted by: Lusubilo A. Mwaijengo

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