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Jellyfish are an invertebrate species of sea dwelling animal that are so named due to their gooey gelatin-like bodies. Despite the name, they are not fish. They belong to the Cnidaria phylum, which includes other simple-bodied marine invertebrates such as sea anemones and corals. The jellyfish is considered simple-bodied because, like its Cnidarian cousins, it has no head, brain, heart, eyes, or ears. Thus it is also lacking in the sensory systems that correspond to these organs.
There are over 2,000 species of jellyfish, or jellies as they are sometimes called. Fossil evidence of these creatures dates back to over 650 million years ago, during the late Proterozoic Era. With so many species of cnidarians floating about, there is bound to be variety in the appearance of their body parts. However, the typical body is composed 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 flap or pump, in order to propel the animal around in the water. Inside the bell are the mouth and stomach cavity. The digestive system is very simple. It takes in food and expels waste through the same opening. The stomach cavity, which can be considered the jellyfish’s “inside,” is lined with cells called gastrodermal cells. These cells are involved in digesting food, and are separated from the “outside,” or epidermis, by a layer of jelly-producing mesoglea. The edge of the bell-shaped body called the rhopalial lappet, and is the location where the tentacles are usually attached.
The jellyfish has no brain, but it does have a simple nervous system called a nerve net, which spreads throughout the epidermis of the animal. When the tentacles come in contact with potential prey, the cells of the nerve net respond by shooting out the many stinging cells contained in the tentacles. This is how the creature reacts to touch stimuli.
Likewise, although the jellyfish does not have sensory organs, per se, it does possess tiny simple sensory structures that allow it to respond to other external stimuli. Ocelli are simple but specialized structures in the body that react to light. Thus the jellyfish, lacking a brain and eyes, can respond to sunlight shining through the water. It should be noted that, although it can perceive light, without eyes it cannot see images. This animal might also have a statolith structure, which allows the animal to respond to gravity.
Jellyfish, because of their stinging tentacles, can be dangerous to humans. Of the more than 2,000 species, roughly 70 are thought to be potentially dangerous to humans. The stings of some of these can even be fatal to humans, and may leave permanent scars on any victim lucky enough to survive. The most dangerous species are the Lion’s Mane jelly, the Sea Nettle jelly, the Sea Wasp jelly, and the Portuguese Man-of-war.