A colored jellyfish.
Jellyfish are a species of invertebrate marine animals named for their sticky, gelatinous bodies. Despite the name, they are not fish. They belong to the phylum Cnidaria, which includes other single-bodied marine invertebrates such as sea anemones and corals. Jellyfish are considered simple-bodied because, like their cnidarian cousins, they don’t have a head, brain, heart, eyes, or ears. Therefore, it is also absent in the sensory systems that correspond to these organs.
Bioluminescent jellyfish often populate the aphotic zone of the ocean and produce their own light.
There are more than 2,000 species of jellyfish, or jellies as they are sometimes called. Fossil evidence of these creatures dates back more than 650 million years, during the late Proterozoic Era. With so many species of cnidarians floating around, there is likely to be variety in the appearance of their body parts. However, the typical body is made up of the bell, the oral arms or feeding arms, and the tentacles. There are some species that do not have tentacles.
The bell of the jellyfish is the smooth, umbrella-shaped body that is designed to pound or pump to propel the animal through the water. Inside the bell are the mouth and stomach cavity. The digestive system is very simple. It receives food and expels waste through the same opening. The stomach cavity, which can be considered the “inside” of the jellyfish, is lined with cells called gastrodermal cells. These cells are involved in the digestion of food and are separated from the “outside,” or epidermis, by a layer of mesoglea that produces gelatin. The edge of the bell-shaped body is called the lapete ropalia, and is where the tentacles usually attach.
Jellyfish do not have a brain, but they do have a simple nervous system called a nerve net, which extends through the epidermis of the animal. When the tentacles come into contact with potential prey, the cells of the nerve net respond by firing the many stinging cells contained in the tentacles. This is how the creature reacts to tactile stimuli.
Also, although jellyfish do not have sensory organs, they do have tiny, simple sensory structures that allow them to respond to other external stimuli. The ocelli are simple but specialized structures in the body that react to light. Therefore, the jellyfish, without a brain or eyes, can respond to sunlight shining on the water. It should be noted that although he can perceive light, without eyes he cannot see images. This animal may also have a statolith structure, which allows it to respond to gravity.
Jellyfish, due to their stinging tentacles, can be dangerous to humans. Of the more than 2,000 species, some 70 are considered potentially dangerous to humans. The bites of some of them can even be fatal to humans and can leave permanent scars on any victims lucky enough to survive. The most dangerous species are the Juba do Leão jellyfish, the nettle jellyfish, the sea vespa jellyfish and the Portuguese man-of-war.