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4. Cnidaria.All of them are aquatic and most are marine organisms. We can find here some of the most beautiful invertebrates and also some of the most dangerous. The animals in this group have stinging tentacles with a characteristic type of cells called cnidocytes or nematocysts. All the members of this phylum can be classified into two different kinds of body shape: the polyp-shaped members and the medusa-shaped members. Polyps are sessile animals that live attached to a surface and have a body with the shape of a bag. Some species live individually, like the anemones or the hydras, but others, like corals, live in colonies of many individuals that remain together when reproduce by budding. Sometimes they cover themselves with a skeleton made of limestone that remains when the animal dies and constitutes the coral reefs, some of them, like The Great Reef Barrier at the East of Australia, are so big that can be seen as far as from the Moon. Medusas, commonly known as jellyfish, look like gelatinous, delicate and transparent animals that swim elegantly by regular contractions of their umbrella-shaped body. Both polyps and jellyfish have radial symmetry and they are carnivores. They capture their preys by means of the tentacles that have cnidocytes. When something gets in contact with a cnidocyte it fires a microscopic structure similar to a harpoon that injects a toxin that immobilizes or kills the prey. Then the tentacles take the prey to the mouth and it is swallowed into a gastrovascular cavity where digestion takes place. The residues are eliminated also by the mouth as these animals have not an anus. Polyps can reproduce asexually by budding or fragmentation and sexually by male and female gametes that swim to produce a zygote and then a larva that eventually will attach to a new surface to develop a new polyp. Jellyfish only reproduce sexually.