The jelyfish is a kind of marine animal belonging to the class Scyphozoa. Tufted umbrella-shaped body, can make the itching of the skin upon contact.
Cnidaria is a phylum consisting of about 9,000 species of simple animals found exclusively in aquatic, mostly marine environment. From the point of etymology, the word Cnidaria comes from the Greek "cnidos" which means "stinging needles". The ability to sting cephalopods who is the origin of the name Cnidaria is typical them. characteristics knidosit, which are specialized cells that they use mainly for capturing prey and defend themselves.
Their bodies consist of mesoglea, a non-living materials like jelly, located between two layers of the epithelium which is usually one cell thick. They have two basic body forms: medusa-swimming and sessile polyps, both radially symmetrical with mouths surrounded by tentacles have knidosit. Both of these forms have one entrance hole that serves as the mouth or anus called an astronomer as well as the body cavity that is used for digesting food and breathing.
Many cephalopods produce colonies that meruapakan single organisms composed of medusa-like zooids or polyps resemble or both. Cephalopods activities are coordinated by neural nets do not centralized and simple receptors. Some Cubozoa and free-swimming Scyphozoa have a sense of balance and there have statokista ropalia, sensing a complex structure that can include the image-forming eyes with a simple lens and retina. All cephalopods reproduce sexually. Many cephalopods have complex life cycle with asexual polyp stage of development and sexual medusa, but some do not have a polyp or do not have a medusa.
Jelyfish stings killed several hundred people in the 20th century, and the box jelyfish is a particularly dangerous. On the other hand, some large jellyfish are considered as good food in the east and south Asia. Coral reefs have long been considered important economically as a fishing spot, protective building on the beach of tidal currents and sea water, and recently as a tourist center. However, they are vulnerable to overfishing, mining, building materials, pollution, and damage caused by tourism.