A Group Of Jellyfish Swimming Underwater On Display In An Aquarium

In the complicated classifications of marine invertebrates, the jellyfish belong to the Phylum of the Cnidarians, which has about 10,000 species, included in the largest group of Coelenterates, together with the Ctenophores, gelatinous organisms without stinging organs.

Poisons aside, one of the most important characteristics of the Cnidarians is the presence of different forms: the octopus (which has nothing to do with the octopus of our reefs, which is a cephalopod mollusk) and the jellyfish. The octopus is fixed to the seabed and has a crown of tentacles facing upwards; the jellyfish, on the contrary, moves in open water, and often has the shape of a bell (called umbrella) with the tentacles on the underside.

The two forms are closely linked: octopus and jellyfish derive from each other and very often the same species has both forms, at different stages of its life.

The famous corals are nothing more than large colonies of polyps, capable of developing an external calcareous skeleton, which forms the basis of the immense tropical coral reefs. They are also Cnidarians, but they belong to a particular group, the Anthozoans, which do not have a very close link with the jellyfish we know.