Before entering fully into the establishment of the meaning of the term cnidarians, it is necessary to know its etymological origin. In this case, we can state that it is the result of the sum of two well differentiated parts:
-The Greek noun “knide”, which can be translated as “nettle”.
-The Latin “-arium”, which is used to indicate “belonging”.

The cnidarians are celentéreos animals presenting cells stinging and often live fixed at the bottom of the sea, in colonies or communities of plankton. The jellyfish and corals belong to this phylum.

Returning to the definition, we can say that cnidarians are coelenterates since they exhibit radiated symmetry and have a single gastrovascular cavity with an outlet to the outside, which functions as anus and mouth simultaneously. On the other hand, stinging cells have the ability to itch.

According to DigoPaul, all cnidarians are aquatic and mostly marine. Their body resembles a sac and they have one or more tentacles around their mouths. Cnidarians do not have a circulatory, respiratory, or excretory system: all of these functions are satisfied by the aforementioned gastrovascular cavity and the ectodermis (the outer layer of the body).

In the ectodermis there are different classes of cells, such as basal cells, sensory cells, and neurons. The anatomy of the cnidarians also includes the gastrodermis (the inner body layer, which defines the gastrovascular cavity) and the mesoglea (located between the ectodermis and the gastrodermis).

Beyond the common characteristics, it is possible to differentiate between about 10,000 species of cnidarians. Some, like polyps, are sessile – they don’t have organs or limbs to support their bodies.

In the phylum of the cnidarians it is also possible to distinguish between several classes, such as the siphozoa, the hydrozoa, the cubozoa and the anthozoa. All classes are believed to have derived from anthozoans, which are the oldest cnidarians.

In addition to what is indicated, we can establish another series of relevant aspects about cnidarians that are worth knowing:
-In this group there are also jellyfish, hydras or anemones.
-They have two layers of cells: the interior, which responds to the name of endoderm, and the exterior, which is called ectoderm.
-From a reproductive point of view, we can establish that they reproduce, both sexually and asexually, through what is known as budding. However, we must not forget the fact that others have what goes by the name of alternating reproduction. This is what takes place when asexual reproduction, which is that of the polyp phase, alternates with sexual reproduction, in the jellyfish phase.
-Hydrozoans are identified because throughout their lives they will alternate what is the polyp phase with the jellyfish.
-The anthozoans, on the other hand, come to be corals and anemones. They always have the shape of polyps and have a kind of pharynx, which is divided into several clearly delimited parts.
-Sphozoans come to be jellyfish.